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

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

CMakeLists.txt

@@ -72,6 +72,7 @@ if(AERON_TESTS)
        URL ${CMAKE_CURRENT_SOURCE_DIR}/cppbuild/googletest-release-1.8.0.zip
        URL_MD5 adfafc8512ab65fd3cf7955ef0100ff5
        PREFIX "${AERON_THIRDPARTY_BINARY_DIR}/gmock"
+       CMAKE_ARGS -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER};-DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}
        BUILD_BYPRODUCTS "${AERON_THIRDPARTY_BINARY_DIR}/gmock/src/gmock-build/googlemock/${CMAKE_CFG_INTDIR}/${CMAKE_STATIC_LIBRARY_PREFIX}gmock${CMAKE_STATIC_LIBRARY_SUFFIX};${AERON_THIRDPARTY_BINARY_DIR}/gmock/src/gmock-build/googlemock/${CMAKE_CFG_INTDIR}/${CMAKE_STATIC_LIBRARY_PREFIX}gmock_main${CMAKE_STATIC_LIBRARY_SUFFIX}"
        INSTALL_COMMAND ""
    )
@@ -95,6 +96,7 @@ if(AERON_BUILD_SAMPLES)
         hdr_histogram
         URL ${CMAKE_CURRENT_SOURCE_DIR}/cppbuild/HdrHistogram_c.zip
         URL_MD5 8d1d6c6df1ee8342c180529a89568081
+        CMAKE_ARGS -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER};-DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}
         PREFIX "${AERON_THIRDPARTY_BINARY_DIR}/hdr_histogram"
         BUILD_BYPRODUCTS "${AERON_THIRDPARTY_BINARY_DIR}/hdr_histogram/src/hdr_histogram-build/src/${CMAKE_STATIC_LIBRARY_PREFIX}hdr_histogram_static${CMAKE_STATIC_LIBRARY_SUFFIX}"
         INSTALL_COMMAND ""
@@ -118,7 +120,8 @@ if(AERON_TESTS)
         google_benchmark
         URL ${CMAKE_CURRENT_SOURCE_DIR}/cppbuild/benchmark-1.4.0.tar.gz
         URL_MD5 ccfaf2cd93ae20191b94f730b945423e
-        CMAKE_ARGS -DBENCHMARK_ENABLE_GTEST_TESTS=OFF;-DBENCHMARK_ENABLE_ASSEMBLY_TESTS=OFF
+        CMAKE_ARGS -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER};-DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER};
+            -DBENCHMARK_ENABLE_GTEST_TESTS=OFF;-DBENCHMARK_ENABLE_ASSEMBLY_TESTS=OFF
         PREFIX "${AERON_THIRDPARTY_BINARY_DIR}/google_benchmark"
         BUILD_BYPRODUCTS "${AERON_THIRDPARTY_BINARY_DIR}/google_benchmark/src/google_benchmark-build/src/${CMAKE_STATIC_LIBRARY_PREFIX}benchmark${CMAKE_STATIC_LIBRARY_SUFFIX}"
         INSTALL_COMMAND ""