Googletest vendoring and install detection for cmake

gtest.cmake

# CMake GoogleTest vendoring helper
# Adapted from https://github.com/snikulov/google-test-examples

include(ExternalProject)
find_package(PkgConfig REQUIRED)

pkg_check_modules(GTEST gtest)

set(GTEST_FORCE_SHARED_CRT ON)
set(GTEST_DISABLE_PTHREADS OFF)

if(MINGW)
    set(GTEST_DISABLE_PTHREADS ON)
endif()

# If GTEST_INCLUDE_DIRS and GTEST_LIBS_DIR are not defined, vendor in google test
if ((NOT DEFINED GTEST_INCLUDE_DIRS) AND (NOT DEFINED GTEST_LIBS_DIR))

message(STATUS "no googletest install found, vendoring into project")

ExternalProject_Add(googletest
    GIT_REPOSITORY https://github.com/google/googletest.git
    GIT_SHALLOW true
    GIT_TAG release-1.8.1
    CMAKE_ARGS -DCMAKE_ARCHIVE_OUTPUT_DIRECTORY_DEBUG:PATH=DebugLibs
    -DCMAKE_ARCHIVE_OUTPUT_DIRECTORY_RELEASE:PATH=ReleaseLibs
    -DCMAKE_CXX_FLAGS=${MSVC_COMPILER_DEFS}
    -Dgtest_force_shared_crt=${GTEST_FORCE_SHARED_CRT}
    -Dgtest_disable_pthreads=${GTEST_DISABLE_PTHREADS}
    -DBUILD_GTEST=ON
    PREFIX "${CMAKE_CURRENT_BINARY_DIR}"
    # Disable install step
    INSTALL_COMMAND ""
    )

# Specify include dir
ExternalProject_Get_Property(googletest source_dir)
set(GTEST_INCLUDE_DIRS ${source_dir}/googletest/include)

# Specify MainTest's link libraries
ExternalProject_Get_Property(googletest binary_dir)
set(GTEST_LIBS_DIR ${CMAKE_CURRENT_BINARY_DIR}/src/googletest-build/googlemock/gtest)

# Use this to gate adding a dependency to the test project (only required when vending)
set(GTEST_VENDORED  "")

endif ()

# Include GTest stuff
message(STATUS "Using googletest from:")
message(STATUS "    GTEST_INCLUDE_DIRS=${GTEST_INCLUDE_DIRS}")
message(STATUS "    GTEST_LIBS_DIR=${GTEST_LIBS_DIR}")

include_directories(${GTEST_INCLUDE_DIRS})
link_directories(${GTEST_LIBS_DIR})