Configure a basic CMake library

Provides 2 targets: `Portduino::Portduino` and `Porduino::PortduinoMain`

Can be used like so:

```cmake
include(FetchContent)
FetchContent_Declare(Portduino
  GIT_REPOSITORY https://github.com/glebm/framework-portduino.git
  GIT_TAG ...
  EXCLUDE_FROM_ALL)
FetchContent_MakeAvailable(Portduino)

...

target_link_libraries(MyProject PRIVATE Portduino::Portduino)

```

.gitignore

@@ -3,3 +3,6 @@
 .vscode/c_cpp_properties.json
 .vscode/launch.json
 .vscode/ipch
+/build/
+/build-*/
+**/__pycache__/

CMakeLists.txt

@@ -1,13 +1,8 @@
 cmake_minimum_required(VERSION 3.17)
-project(Portduino)
+project(Portduino VERSION 0.0.1 LANGUAGES C CXX)
 
-set(CMAKE_CXX_STANDARD 14)
-
-include_directories(cores/arduino/api)
-include_directories(cores/portduino)
-include_directories(cores/portduino/FS)
-
-add_executable(Portduino
+add_library(
+        Portduino
         cores/arduino/api/deprecated/Client.h
         cores/arduino/api/deprecated/HardwareSerial.h
         cores/arduino/api/deprecated/IPAddress.h
@@ -50,17 +45,103 @@ add_executable(Portduino
         cores/portduino/Arduino.h
         cores/portduino/itoa.cpp
         cores/portduino/dtostrf.c
-        cores/portduino/main.cpp
         cores/portduino/linux/millis.cpp
         cores/portduino/simulated/SimCommon.cpp
         cores/portduino/Utility.cpp
         cores/portduino/Utility.h
         cores/portduino/simulated/SimHardwareSPI.cpp
-        example/test.cpp
-       	cores/portduino/simulated/SimHardwareI2C.cpp
-       	cores/portduino/simulated/SimHardwareI2C.h
-       	cores/portduino/linux/LinuxCommon.cpp
-       	cores/portduino/linux/LinuxSerial.cpp
+        cores/portduino/simulated/SimHardwareI2C.cpp
+        cores/portduino/simulated/SimHardwareI2C.h
+        cores/portduino/linux/LinuxCommon.cpp
+        cores/portduino/linux/LinuxSerial.cpp
         cores/portduino/linux/LinuxSerial.h
         cores/portduino/linux/LinuxHardwareI2C.cpp
-       	cores/portduino/linux/LinuxHardwareI2C.h)
+        cores/portduino/linux/LinuxHardwareI2C.h)
+set_target_properties(Portduino PROPERTIES PUBLIC_HEADER
+        cores/arduino/api/ArduinoAPI.h
+        cores/arduino/api/Binary.h
+        cores/arduino/api/Client.h
+        cores/arduino/api/Common.h
+        cores/arduino/api/Compat.h
+        cores/arduino/api/HardwareI2C.h
+        cores/arduino/api/HardwareSerial.h
+        cores/arduino/api/HardwareSPI.h
+        cores/arduino/api/Interrupts.h
+        cores/arduino/api/IPAddress.h
+        cores/arduino/api/itoa.h
+        cores/arduino/api/PluggableUSB.h
+        cores/arduino/api/Printable.h
+        cores/arduino/api/RingBuffer.h
+        cores/arduino/api/Server.h
+        cores/arduino/api/Stream.h
+        cores/arduino/api/String.h
+        cores/arduino/api/Udp.h
+        cores/arduino/api/USBAPI.h
+        cores/arduino/api/WCharacter.h
+        cores/portduino/Arduino.h
+        cores/portduino/FS/FS.h
+        cores/portduino/FS/FSImpl.h
+        cores/portduino/FS/PortduinoFS.h
+        cores/portduino/FS/vfs_api.h
+        cores/portduino/linux/gpio/LinuxGPIOPin.h
+        cores/portduino/linux/LinuxHardwareI2C.h
+        cores/portduino/linux/LinuxSerial.h
+        cores/portduino/linux/PosixFile.h
+        cores/portduino/logging.h
+        cores/portduino/PortduinoGPIO.h
+        cores/portduino/simulated/SimHardwareI2C.h
+        cores/portduino/SPIChip.h
+        cores/portduino/Utility.h
+        cores/portduino/WString.h
+)
+target_include_directories(Portduino
+        SYSTEM PUBLIC
+        $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/cores/arduino/api>
+        $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/cores/portduino>
+        $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/cores/portduino/FS>
+        $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
+)
+add_library(Portduino::Portduino ALIAS Portduino)
+
+add_library(PortduinoMain
+        cores/portduino/main.cpp
+)
+target_link_libraries(PortduinoMain PRIVATE Portduino)
+add_library(Portduino::PortduinoMain ALIAS PortduinoMain)
+
+
+install(
+        TARGETS
+        Portduino PortduinoMain
+        EXPORT
+        PortduinoTargets
+        PUBLIC_HEADER
+        CONFIGURATIONS Release
+)
+
+include(CMakePackageConfigHelpers)
+
+configure_package_config_file(
+        "${PROJECT_SOURCE_DIR}/cmake/PortduinoConfig.cmake.in"
+        "${PROJECT_BINARY_DIR}/PortduinoConfig.cmake"
+        INSTALL_DESTINATION "${CMAKE_INSTALL_DATAROOTDIR}/Portduino/cmake"
+)
+write_basic_package_version_file(
+        "${PROJECT_BINARY_DIR}/PortduinoConfigVersion.cmake"
+        VERSION ${PROJECT_VERSION}
+        COMPATIBILITY SameMajorVersion
+)
+
+install(
+        EXPORT PortduinoTargets
+        FILE PortduinoTargets.cmake
+        NAMESPACE Portduino::
+        DESTINATION "${CMAKE_INSTALL_DATAROOTDIR}/Portduino/cmake"
+)
+
+install(
+        FILES
+                "${PROJECT_BINARY_DIR}/PortduinoConfig.cmake"
+                "${PROJECT_BINARY_DIR}/PortduinoConfigVersion.cmake"
+        DESTINATION "${CMAKE_INSTALL_DATAROOTDIR}/Portduino/cmake"
+)

cmake/PortduinoConfig.cmake.in

@@ -0,0 +1,5 @@
+@PACKAGE_INIT@
+
+check_required_components(Portduino)
+
+include("${CMAKE_CURRENT_LIST_DIR}/@[email protected]")