[c] Support for cmake ExternalProject

[prabhu]

https://cmake.org/cmake/help/git-master/module/ExternalProject.html

Example:

https://github.com/CastXML/CastXMLSuperbuild/blob/master/CMakeLists.txt

set(llvm_cmake_sha256 acfecb615d41c5b1a0a31e15324994ca06f7a3f37d8958d719b20de0d217b71b)
  ExternalProject_Add(llvm-cmake
    URL "https://github.com/llvm/llvm-project/releases/download/llvmorg-${llvm_folder}/cmake-${llvm_version}.src.tar.xz"
    URL_HASH SHA256=${llvm_cmake_sha256}
    CONFIGURE_COMMAND ""
    BUILD_COMMAND ""
    INSTALL_COMMAND ""
    ${download_extract_timestamp_flag}
    LOG_BUILD 0
    SOURCE_DIR ${CMAKE_CURRENT_BINARY_DIR}/llvm-prefix/src/cmake
    )

  set(llvm_sha256 fa6db8951f5ef576ac6bad43d5e1ed83962754538c998fbfa0397cd4521abc00)
  ExternalProject_Add(llvm
    URL "https://github.com/llvm/llvm-project/releases/download/llvmorg-${llvm_folder}/llvm-${llvm_version}.src.tar.xz"
    URL_HASH SHA256=${llvm_sha256}
    DEPENDS llvm-cmake
    CMAKE_ARGS -Wno-dev
    CMAKE_GENERATOR "${CMAKE_GENERATOR}"
    CMAKE_CACHE_ARGS
      -DCMAKE_CXX_COMPILER:FILEPATH=${CMAKE_CXX_COMPILER}
      -DCMAKE_CXX_STANDARD:STRING=${CMAKE_CXX_STANDARD}
      -DCMAKE_C_COMPILER:FILEPATH=${CMAKE_C_COMPILER}
      -DLLVM_STATIC_LINK_CXX_STDLIB:BOOL=ON
      -DCMAKE_BUILD_TYPE:STRING=${CMAKE_BUILD_TYPE}
      -DCMAKE_INSTALL_PREFIX:PATH=<INSTALL_DIR>
      -DLLVM_INSTALL_PREFIX:PATH=<INSTALL_DIR>
      -DLLVM_ENABLE_TERMINFO:BOOL=OFF
      -DLLVM_INCLUDE_TESTS:BOOL=OFF
      -DLLVM_INCLUDE_BENCHMARKS:BOOL=OFF
      -DLLVM_INCLUDE_DOCS:BOOL=OFF
      -DLLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN:BOOL=ON
      ${osx_args}
    ${verbose_command}
    ${download_extract_timestamp_flag}
    LOG_BUILD 0
    INSTALL_DIR ${CMAKE_CURRENT_BINARY_DIR}/llvm
    )