Set C standard, removed unused includes

CMakeLists.txt

 cmake_minimum_required(VERSION 3.5)
+set (CMAKE_C_STANDARD 99)
 set (CMAKE_CXX_STANDARD 98)
 project(mbed-os)
 
@@ -16,7 +17,6 @@ include_directories(
         ${CMAKE_CURRENT_SOURCE_DIR}/targets/TARGET_NXP/TARGET_LPC176X
         ${CMAKE_CURRENT_SOURCE_DIR}/targets/TARGET_NXP/TARGET_LPC176X/TARGET_MBED_LPC1768
         ${CMAKE_CURRENT_SOURCE_DIR}/targets/TARGET_NXP/TARGET_LPC176X/device
-        ${CMAKE_CURRENT_SOURCE_DIR}/targets/TARGET_NXP/TARGET_LPC176X/device/TOOLCHAIN_GCC_ARM
 )
 file(GLOB_RECURSE MBED_HEADERS ${CMAKE_CURRENT_SOURCE_DIR}/*.h)
 file(GLOB_RECURSE MBED_SOURCE ${CMAKE_CURRENT_SOURCE_DIR}/*.c ${CMAKE_CURRENT_SOURCE_DIR}/*.cpp)
@@ -31,6 +31,5 @@ add_custom_command(
 add_compile_options(-include ${CMAKE_CURRENT_SOURCE_DIR}/mbed_config.h)
 
 add_library(mbed-os STATIC ${MBED_SOURCE} ${MBED_HEADERS} ${STARTUP_OBJECT})
-set_target_properties(mbed-os PROPERTIES LINKER_LANGUAGE C)
 
 target_include_directories(mbed-os PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})