add separate cxx options for gcc (#80)

* add separate cxx options for gcc

* remove warning write-strings

Author: JojoS62

tools/cmake/mbed_toolchain.cmake

@@ -121,6 +121,7 @@ set(CMAKE_CXX_USE_RESPONSE_FILE_FOR_LIBRARIES 1)
 set(link_options "")
 set(common_options "")
 set(c_cxx_compile_options "") # compile options only for C/CXX
+set(cxx_compile_options "") # compile options only for CXX
 set(asm_compile_options "") # compile options only for ASM
 
 include(toolchains/${MBED_TOOLCHAIN})
@@ -140,7 +141,7 @@ endmacro(list_to_space_separated)
 
 # set toolchain flags with CMake (INIT variables will be picked up on first run)
 list_to_space_separated(CMAKE_C_FLAGS_INIT ${common_options} ${c_cxx_compile_options})
-set(CMAKE_CXX_FLAGS_INIT ${CMAKE_C_FLAGS_INIT})
+list_to_space_separated(CMAKE_CXX_FLAGS_INIT ${common_options} ${c_cxx_compile_options} ${cxx_compile_options})
 list_to_space_separated(CMAKE_ASM_FLAGS_INIT ${common_options} ${asm_compile_options})
 list_to_space_separated(CMAKE_EXE_LINKER_FLAGS_INIT ${link_options})
 

tools/cmake/toolchains/GCC_ARM.cmake

@@ -43,6 +43,10 @@ list(APPEND common_options
     "-g3"
 )
 
+list(APPEND cxx_compile_options
+    "-Wno-register"
+)
+
 # Configure the toolchain to select the selected C library
 function(mbed_set_c_lib target lib_type)
     if (${lib_type} STREQUAL "small")