Add SetFreeRDPCMakeInstallDir function to abstract away platform differences

Different platforms/systems may install CMake modules into different places. For
instance, FreeBSD will install modules into ${PREFIX}/share/cmake/Modules while
Linux distributions might install into ${PREFIX}/lib/cmake.

This leaves us with a useful abstraction- consumers only need to care about
the name of their subdirectory, and only one place needs to be patched to change
where this subdirectory lands.

CMakeLists.txt

@@ -68,6 +68,7 @@ include(GNUInstallDirsWrapper)
 include(CMakePackageConfigHelpers)
 include(InstallFreeRDPMan)
 include(GetGitRevisionDescription)
+include(SetFreeRDPCMakeInstallDir)
 
 # Soname versioning
 set(BUILD_NUMBER 0)

cmake/SetFreeRDPCMakeInstallDir.cmake

@@ -0,0 +1,7 @@
+function(SetFreeRDPCMakeInstallDir SETVAR subdir)
+	if(FREEBSD)
+		set(${SETVAR} "${CMAKE_INSTALL_DATAROOTDIR}/cmake/Modules/${subdir}" PARENT_SCOPE)
+	else()
+		set(${SETVAR} "${CMAKE_INSTALL_LIBDIR}/cmake/${subdir}" PARENT_SCOPE)
+	endif()
+endfunction()