cmake: fix StdFilesystem detection

cmake: fix StdFilesystem detection

before this change, we fails to detect std::filesystem with clang++,
because  the cmake project created by try_compile() only expands
following options passed from its caller:
- COMPILE_DEFINITIONS
- INCLUDE_DIRECTORIES
- LINK_DIRECTORIES
- LINK_LIBRARIES

which do not include CMAKE_CXX_FLAGS, so either we need to (ab)use
COMPILE_DEFINITIONS for passing -std=c++17, or we can change the
CMAKE_CXX_FLAGS in the parent env, as it turns out the created cmake
project does inherit this flag from current project. in this change,
we use the COMPILE_DEFINITIONS approach: simpler this way. and we
can drop it once cmake 3.8 is required.

Signed-off-by: Kefu Chai <kchai@redhat.com>