[BUG] prefab should set LINKER_LANGUAGE property for static libraries

[madebr]

Describe the bug When a static library uses a C++ STL, the linker language of a library should be set to CXX.

To Reproduce Steps to reproduce the behavior:

1. Package a C++ static library using prefab (let's name it staticmodule::staticlib). Make sure this library uses symbols from the STL. e.g. std::string, ...
2. Use the prefab module from step 1 in a C project:

   cmake_minimum_required(VERSION 3.0)
   project(mygame)
   find_package(staticmodule REQUIRED CONFIG)
   add_library(mygame SHARED mygame.c)
   target_link_libraries(mygame PRIVATE staticmodule::staticlib)

3. Building the mygame library will fail because the C linker will not find the c++ STL symbols.

Expected behavior Linking just works.

Additional context Prefab should add a LINKER_LANGUAGE property: e.g.

add_library(SDL2::SDL2-static STATIC IMPORTED)
set_target_properties(SDL2::SDL2-static PROPERTIES
    IMPORTED_LOCATION "/home/maarten/.gradle/caches/transforms-3/3d84dffbc80dce10e99f69ce2cefa8de/transformed/SDL2-2.25.0/prefab/modules/SDL2-static/libs/android.arm64-v8a/libSDL2.a"
    INTERFACE_INCLUDE_DIRECTORIES "/home/maarten/.gradle/caches/transforms-3/3d84dffbc80dce10e99f69ce2cefa8de/transformed/SDL2-2.25.0/prefab/modules/SDL2-static/include"
    INTERFACE_LINK_LIBRARIES "-ldl;-lGLESv1_CM;-lGLESv2;-llog;-landroid;-lOpenSLES"
    LINKER_LANGUAGE CXX   # This line is currently missing
)

[DanAlbert]

I suspect this is a trivial fix since the abi.json already describes the STL requirement.

[madebr]

As an aside, it would also be useful to check inside the SDL2Config.cmake config file whether the CXX language is actually enabled. Because if it is not, the build process will fail at a later time. See e.g. this issue I created earlier this year.

I think the cmake config file might need something like:

include(CheckLanguage)
check_language(CXX)
if(NOT CMAKE_CXX_COMPILER)
    message(WARNING "SDL2 requires CXX, but it not enabled.")
endif()

CheckLanguage documentation

Or skip the warning and do enable_language(CXX) yourself.

[madebr]

I think, instead of LINKER_LANGUAGE, the imported targets should set IMPORTED_LINK_INTERFACE_LANGUAGES.