[cmake] Added module BuildExternalProject.

https://github.com/Sbte/BuildExternalProject/commit/ce1a70996aa538aac17a6faf07db487c3a238838

scripts/cmake/BuildExternalProject.cmake

+macro (BuildExternalProject_find_package target)
+  set(build_dir ${CMAKE_BINARY_DIR}/external/build_${target})
+
+  # Set CMake prefix path so we can look there for the module
+  set(_CMAKE_PREFIX_PATH ${CMAKE_PREFIX_PATH})
+  mark_as_advanced(_CMAKE_PREFIX_PATH)
+  list(APPEND CMAKE_PREFIX_PATH ${build_dir})
+
+  find_package(${target} MODULE QUIET)
+  if (NOT ${target}_FOUND)
+    # Look for config version if there was no module
+    find_package(${target} CONFIG REQUIRED HINTS ${build_dir} NO_DEFAULT_PATH)
+  endif()
+
+  # Set CMake prefix path back to what it was
+  set(CMAKE_PREFIX_PATH ${_CMAKE_PREFIX_PATH})
+  unset(_CMAKE_PREFIX_PATH)
+endmacro()
+
+function (BuildExternalProject target)
+  set(build_dir ${CMAKE_BINARY_DIR}/external/build_${target})
+
+  message(STATUS "Building ${target}")
+
+  file(MAKE_DIRECTORY ${build_dir})
+
+  set(CMAKE_LIST_CONTENT "
+    cmake_minimum_required(VERSION 2.8.2)
+
+    include(ExternalProject)
+    ExternalProject_add(${target}
+      PREFIX ${build_dir}
+      CMAKE_ARGS -DCMAKE_INSTALL_PREFIX=<INSTALL_DIR>
+      LOG_DOWNLOAD ON
+      LOG_UPDATE ON
+      LOG_CONFIGURE ON
+      LOG_BUILD ON
+      LOG_TEST ON
+      LOG_INSTALL ON
+      ${ARGN}
+      )
+
+    add_custom_target(build_${target})
+    add_dependencies(build_${target} ${target})
+    ")
+
+  if (EXISTS ${build_dir}/CMakeLists.txt)
+    file(SHA256 ${build_dir}/CMakeLists.txt file_sha)
+    string(SHA256 new_sha "${CMAKE_LIST_CONTENT}")
+
+    if (NOT file_sha STREQUAL new_sha)
+      file(WRITE ${build_dir}/CMakeLists.txt "${CMAKE_LIST_CONTENT}")
+      BuildExternalProject_configure(${build_dir})
+    endif()
+  else()
+    file(WRITE ${build_dir}/CMakeLists.txt "${CMAKE_LIST_CONTENT}")
+    BuildExternalProject_configure(${build_dir})
+  endif()
+
+  BuildExternalProject_build(${build_dir})
+
+  message(STATUS "Finished building ${target}")
+endfunction()
+
+function (BuildExternalProject_configure build_dir)
+  execute_process(COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" .
+    RESULT_VARIABLE result
+    WORKING_DIRECTORY ${build_dir}
+    OUTPUT_QUIET
+    )
+
+  if(result)
+    message(FATAL_ERROR "CMake step for external project failed: ${result}")
+  endif()
+endfunction()
+
+function (BuildExternalProject_build build_dir)
+  execute_process(COMMAND ${CMAKE_COMMAND} --build .
+    RESULT_VARIABLE result
+    WORKING_DIRECTORY ${build_dir}
+    OUTPUT_QUIET
+    )
+
+  if(result)
+    message(FATAL_ERROR "Build step for external project failed: ${result}")
+  endif()
+endfunction()
\ No newline at end of file