Enable build on Mac OS X with clang

* Add build target 'darwin'
* Modify compiler options to work with clang
* Support 'jerry-libc' on Mac OS X

For MPU target build, install `gcc-arm-none-eabi` tool chain using `Homebrew` as following.
````
brew tap PX4/homebrew-px4
brew update
brew install gcc-arm-none-eabi
````
https://pixhawk.org/dev/toolchain_installation_mac

JerryScript-DCO-1.0-Signed-off-by: Sung-Jae Lee sjlee@mail.com

jerry-libc/CMakeLists.txt

@@ -40,6 +40,8 @@ set(COMPILE_FLAGS_LIBC "${COMPILE_FLAGS_JERRY} ${C_FLAGS_JERRY}")
  # Platform-specific
   # Linux
    set(DEFINES_LIBC_LINUX __TARGET_HOST)
+  # Darwin
+   set(DEFINES_LIBC_DARWIN __TARGET_HOST)
   # MCU
    set(DEFINES_LIBC_MCU __TARGET_MCU)
    # stm32f3
@@ -57,6 +59,9 @@ set(COMPILE_FLAGS_LIBC "${COMPILE_FLAGS_JERRY} ${C_FLAGS_JERRY}")
    # Linux
      set(INCLUDE_THIRD_PARTY_LINUX )
 
+   # Darwin
+     set(INCLUDE_THIRD_PARTY_DARWIN )
+
    # MCU
     # STM32F3
      set(INCLUDE_THIRD_PARTY_MCU_STM32F3
@@ -74,6 +79,9 @@ set(COMPILE_FLAGS_LIBC "${COMPILE_FLAGS_JERRY} ${C_FLAGS_JERRY}")
   # Linux
    file(GLOB SOURCE_LIBC_LINUX target/linux/*.c target/linux/*.S)
 
+  # Darwin
+   file(GLOB SOURCE_LIBC_DARWIN target/darwin/*.c target/darwin/*.S)
+
   # MCU
    # stm32f3
     file(GLOB SOURCE_LIBC_MCU_STM32F3 target/mcu-stubs/*.c target/mcu-stubs/*.S)