Change LIBDEFLATE_STATIC back to LIBDEFLATE_DLL

Undo 8a26b31 ("Fix libdeflate.pc on Windows") which required
Windows users to define LIBDEFLATE_STATIC when linking to the static
library, instead of LIBDEFLATE_DLL when linking to the DLL.
__declspec(dllimport) is actually optional, so the cleanest solution to
this mess seems to be to use the original LIBDEFLATE_DLL approach, and
to omit it from libdeflate.pc since it is optional anyway.

Resolves ebiggers#259

CMakeLists.txt

@@ -189,7 +189,6 @@ if(LIBDEFLATE_BUILD_STATIC_LIB)
                           OUTPUT_NAME ${STATIC_LIB_NAME}
                           PUBLIC_HEADER libdeflate.h)
     target_include_directories(libdeflate_static PUBLIC ${LIB_INCLUDE_DIRS})
-    target_compile_definitions(libdeflate_static PUBLIC LIBDEFLATE_STATIC)
     target_compile_definitions(libdeflate_static PRIVATE ${LIB_COMPILE_DEFINITIONS})
     target_compile_options(libdeflate_static PRIVATE ${LIB_COMPILE_OPTIONS})
     list(APPEND LIB_TARGETS libdeflate_static)
@@ -209,6 +208,7 @@ if(LIBDEFLATE_BUILD_SHARED_LIB)
                           C_VISIBILITY_PRESET hidden
                           SOVERSION 0)
     target_include_directories(libdeflate_shared PUBLIC ${LIB_INCLUDE_DIRS})
+    target_compile_definitions(libdeflate_shared PUBLIC LIBDEFLATE_DLL)
     target_compile_definitions(libdeflate_shared PRIVATE ${LIB_COMPILE_DEFINITIONS})
     target_compile_options(libdeflate_shared PRIVATE ${LIB_COMPILE_OPTIONS})
     target_link_libraries(libdeflate_shared PRIVATE ${LIB_LINK_LIBRARIES})

libdeflate.h

@@ -17,20 +17,25 @@ extern "C" {
 #include <stdint.h>
 
 /*
- * On Windows, you must define LIBDEFLATE_STATIC if you are linking to the
- * static library version of libdeflate instead of the DLL.  On other platforms,
- * LIBDEFLATE_STATIC has no effect.
+ * Users of libdeflate.dll on Windows can define LIBDEFLATE_DLL to cause
+ * __declspec(dllimport) to be used.  This should be done when it's easy to do.
+ * Otherwise it's fine to skip it, since it is a very minor performance
+ * optimization that is irrelevant for most use cases of libdeflate.
  */
-#ifdef _WIN32
-#  if defined(LIBDEFLATE_STATIC)
-#    define LIBDEFLATEEXPORT
-#  elif defined(BUILDING_LIBDEFLATE)
-#    define LIBDEFLATEEXPORT	__declspec(dllexport)
-#  else
-#    define LIBDEFLATEEXPORT	__declspec(dllimport)
+#undef LIBDEFLATEEXPORT
+#ifdef LIBDEFLATE_DLL
+#  if defined(_WIN32) || defined(__CYGWIN__)
+#    ifdef BUILDING_LIBDEFLATE
+#      define LIBDEFLATEEXPORT	__declspec(dllexport)
+#    else
+#      define LIBDEFLATEEXPORT	__declspec(dllimport)
+#    endif
+#  elif defined(__GNUC__) && defined(BUILDING_LIBDEFLATE)
+#    define LIBDEFLATEEXPORT	__attribute__((visibility("default")))
 #  endif
-#else
-#  define LIBDEFLATEEXPORT	__attribute__((visibility("default")))
+#endif
+#ifndef LIBDEFLATEEXPORT
+#  define LIBDEFLATEEXPORT
 #endif
 
 #if defined(BUILDING_LIBDEFLATE) && defined(__GNUC__) && defined(__i386__)

libdeflate.pc.in

@@ -8,4 +8,11 @@ Description: Fast implementation of DEFLATE, zlib, and gzip
 Version: @PROJECT_VERSION@
 Libs: -L${libdir} -ldeflate
 Cflags: -I${includedir}
-Cflags.private: -DLIBDEFLATE_STATIC
+
+# Note: this library's public header allows LIBDEFLATE_DLL to be defined when
+# linking to the DLL on Windows, to make __declspec(dllimport) be used.
+# However, the only way to define a shared-library-only flag in a pkgconfig file
+# is to use the weird workaround of unconditionally defining it in Cflags, then
+# undefining it in Cflags.private.  Just don't bother with this, since
+# __declspec(dllimport) is optional anyway.  It is a very minor performance
+# optimization that is irrelevant for most use cases of libdeflate.