diff --git a/builder/actions/cmake.py b/builder/actions/cmake.py
index 5d1d31335..4031aee7d 100644
--- a/builder/actions/cmake.py
+++ b/builder/actions/cmake.py
@@ -2,8 +2,10 @@
 # SPDX-License-Identifier: Apache-2.0.
 
 import os
+import pprint
 import re
 import shutil
+from collections import defaultdict
 from functools import lru_cache, partial
 from pathlib import Path
 
@@ -100,6 +102,29 @@ def _project_dirs(env, project):
     return source_dir, build_dir, install_dir
 
 
+def _merge_cmake_lang_flags(cmake_args):
+    print("=== _merge_cmake_lang_flags: cmake args")
+    pprint.pprint(cmake_args, indent=4, depth=4)
+    pattern = re.compile(r'''-D(CMAKE_C(?:XX)?_FLAGS)=["']?([^"']+)''')
+
+    new_cmake_args = []
+
+    cmake_lang_flags = defaultdict(list)
+    for arg in cmake_args:
+        m = pattern.match(arg)
+        if m:
+            cmake_lang_flags[m.group(1)].append(m.group(2))
+        else:
+            new_cmake_args.append(arg)
+
+    pprint.pprint(cmake_lang_flags, indent=4, depth=4)
+
+    for (k, v) in cmake_lang_flags.items():
+        new_cmake_args.append('-D{}={}'.format(k, ' '.join(v)))
+
+    return new_cmake_args
+
+
 def _build_project(env, project, cmake_extra, build_tests=False, args_transformer=None, coverage=False):
     sh = env.shell
     config = project.get_config(env.spec)
@@ -135,6 +160,7 @@ def _build_project(env, project, cmake_extra, build_tests=False, args_transforme
 
     # Set compiler flags
     compiler_flags = []
+    c_path = None
     if toolchain.compiler != 'default' and toolchain.compiler != 'msvc' and not toolchain.cross_compile:
         c_path = toolchain.compiler_path()
         cxx_path = toolchain.cxx_compiler_path()
@@ -163,6 +189,7 @@ def _build_project(env, project, cmake_extra, build_tests=False, args_transforme
     # as off. Without UniqueList cmake will treat it as on.
     cmake_args += project.cmake_args(env)
     cmake_args += cmake_extra
+    cmake_args += ['-DCMAKE_VERBOSE_MAKEFILE=ON']
     if coverage:
         if c_path and "gcc" in c_path:
             # Tell cmake to add coverage related configuration. And make sure GCC is used to compile the project.
@@ -177,6 +204,13 @@ def _build_project(env, project, cmake_extra, build_tests=False, args_transforme
         else:
             raise Exception('--coverage only support GCC as compiler. Current compiler is: {}'.format(c_path))
 
+    # If there are multiple of the same -DCMAKE_<LANG>_FLAGS arguments, CMake takes only the last one.
+    # Since -DCMAKE_<LANG>_FLAGS can be set in multiple places (e.g. in a default config for a specific platform or
+    # compiler, in a user project's config, in this Python module, etc.), we should merge language flags into one per
+    # language.
+    cmake_args = _merge_cmake_lang_flags(cmake_args)
+    print("=== _build_project: cmake_args: {}".format(cmake_args))
+
     # Allow caller to programmatically tweak the cmake_args,
     # as a last resort in case data merging wasn't working out
     if args_transformer:
@@ -197,6 +231,7 @@ def _build_project(env, project, cmake_extra, build_tests=False, args_transforme
     # configure
     sh.exec(*toolchain.shell_env, cmake, cmake_args, working_dir=working_dir, check=True)
 
+    print("=== toolchain.host is {}".format(toolchain.host))
     # build & install
     sh.exec(*toolchain.shell_env, cmake, "--build", project_build_dir, "--config",
             build_config, "--target", "install", working_dir=working_dir, check=True)
diff --git a/builder/core/data.py b/builder/core/data.py
index c1e226666..70a507fe9 100644
--- a/builder/core/data.py
+++ b/builder/core/data.py
@@ -508,6 +508,9 @@ class PKG_TOOLS(Enum):
             '3': {
                 'c': "clang-3.9",
                 'cxx': "clang++-3.9",
+                # specific version number in use
+                'releases': ['3.9'],
+                'cmake_args': ['-DAWS_WARNINGS_ARE_ERRORS=OFF'],
             },
             '6': {
                 'c': "clang-6.0",
diff --git a/builder/core/toolchain.py b/builder/core/toolchain.py
index c33fc9815..2a3e160b1 100644
--- a/builder/core/toolchain.py
+++ b/builder/core/toolchain.py
@@ -14,7 +14,6 @@ def _compiler_version(cc):
     if current_os() != 'windows':
         result = util.run_command(cc, '--version', quiet=True)
         lines = result.output.split('\n')
-
         for text in lines:
             # Apple clang
             m = re.match(r'Apple (LLVM|clang) version (\d+)', text)
@@ -45,15 +44,24 @@ def _find_compiler_tool(name, versions):
     for version in versions:
         for pattern in ('{name}-{version}', '{name}-{version}.0'):
             exe = pattern.format(name=name, version=version)
+            print("version: {}".format(exe))
             path = util.where(exe, resolve_symlinks=False)
             if path:
                 return path, version
     return None, None
 
 
-def _clang_versions():
-    versions = [v for v in COMPILERS['clang']
-                ['versions'].keys() if v != 'default']
+def _clang_versions(version=None):
+    versions = []
+    all_versions = [v for v in COMPILERS['clang']
+                    ['versions'].keys() if v != 'default']
+    if version:
+        if version in all_versions:
+            versions = [version]
+            if 'releases' in COMPILERS['clang']['versions'][str(version)]:
+                versions += COMPILERS['clang']['versions'][str(version)]['releases']
+    else:
+        versions += all_versions
     versions.sort()
     versions.reverse()
     return versions
@@ -181,7 +189,7 @@ def find_gcc_tool(name, version=None):
     def find_llvm_tool(name, version=None):
         """ Finds clang, clang-tidy, lld, etc at a specific version, or the
         latest one available """
-        versions = [version] if version else _clang_versions()
+        versions = _clang_versions(version)
         return _find_compiler_tool(name, versions)
 
     @staticmethod