cmake/modules: replace ";" with " " in compile flags

cmake/modules: replace ";" with " " in compile flags

as the flags should be a string, not a list

tested using

cmake -DCMAKE_LINKER=gold -DWITH_ASAN=ON -DWITH_ASAN_LEAK=ON \
-DCMAKE_BUILD_TYPE=Debug

on ubuntu xenial

without this change, cmake will fail with following warning:

CMake Error at src/CMakeLists.txt:112 (message):
  The compiler /usr/bin/c++ has no C++17 support.

-- Configuring incomplete, errors occurred!

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