Check if -static-{libgcc,libstdc++} flags are supported before passing them

This fixes build on FreeBSD (with Clang).

Author: arrowd

src/CMakeLists.txt

@@ -14,17 +14,25 @@ endif()
 
 set(_target_name linuxdeploy-plugin-appimage)
 
+include(CheckCXXCompilerFlag)
+check_cxx_compiler_flag(-static-libstdc++ STATIC_LIBSTDCXX_SUPPORTED)
+check_cxx_compiler_flag(-static-libgcc STATIC_LIBGCC_SUPPORTED)
+
 # main executable
 add_executable(${_target_name} main.cpp)
 
 target_link_libraries(${_target_name} args)
 
 set(_cflags
     -static
-    -static-libstdc++
-    -static-libgcc
     -flto
 )
+if(STATIC_LIBSTDCXX_SUPPORTED)
+    list(APPEND _cflags -static-libstdc++)
+endif()
+if(STATIC_LIBGCC_SUPPORTED)
+    list(APPEND _cflags -static-libgcc)
+endif()
 target_compile_options(${_target_name} PUBLIC ${_cflags})
 target_link_options(${_target_name} PUBLIC ${_cflags})