[C++] Pass C and C++ compilers to subprojects.

If a compiler set different from CC and CXX is used to build SBE
and tests are enabled by using CMAKE_C_COMPILER and CMAKE_CXX_COMPILER,
SBE source and the tests are built using different compilers.

Author: denizevrenci

CMakeLists.txt

@@ -52,6 +52,7 @@ if(SBE_TESTS)
         URL ${CMAKE_CURRENT_SOURCE_DIR}/cppbuild/googletest-release-1.8.0.zip
         URL_MD5 adfafc8512ab65fd3cf7955ef0100ff5
         PREFIX "${SBE_THIRDPARTY_BINARY_DIR}/gtest"
+        CMAKE_ARGS -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER};-DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}
         BUILD_BYPRODUCTS "${SBE_THIRDPARTY_BINARY_DIR}/gtest/src/gtest-build/googlemock/${CMAKE_CFG_INTDIR}/${CMAKE_STATIC_LIBRARY_PREFIX}gmock${CMAKE_STATIC_LIBRARY_SUFFIX};${SBE_THIRDPARTY_BINARY_DIR}/gtest/src/gtest-build/googlemock/${CMAKE_CFG_INTDIR}/${CMAKE_STATIC_LIBRARY_PREFIX}gmock_main${CMAKE_STATIC_LIBRARY_SUFFIX}"
         INSTALL_COMMAND ""
         LOG_CONFIGURE ON