CMake: modulemap override in build interface (#1539)

The modulemap override should only be added in the build interface,
where we don't copy the modulemap and header files. The install
interface installs the modulemap and header files to a location where
the compiler will see it, so passing the override results in the
compiler seeing both modulemaps defining the same module and then
failing. This moves the override to only be part of the build interface.

Author: etcwilde

Sources/_FoundationCShims/CMakeLists.txt

@@ -20,7 +20,7 @@ add_library(_FoundationCShims STATIC
 target_include_directories(_FoundationCShims PUBLIC include)
 
 target_compile_options(_FoundationCShims INTERFACE
-  "$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xcc -fmodule-map-file=${CMAKE_CURRENT_SOURCE_DIR}/include/module.modulemap>")
+  "$<BUILD_INTERFACE:$<$<COMPILE_LANGUAGE:Swift>:SHELL:-Xcc -fmodule-map-file=${CMAKE_CURRENT_SOURCE_DIR}/include/module.modulemap>>")
 
 set_property(GLOBAL APPEND PROPERTY SWIFT_FOUNDATION_EXPORTS _FoundationCShims)