fix a bug introduced by previous lua-bcc build fix commit

fix a bug introduced by previous lua-bcc build fix commit

The bug does not show up in my previous test probably because
I did not clean up the build directory and did not have a clean cmake.

In gcc 4.8.5 environment where -no-pie is not supported,
a clean cmake will trigger the issue as subsequent other
cxx compile test will fail due to additional -no-pie flag.

The fix is to reset the CMAKE_REQUIRED_FLAGS to its previous value
after the specific test.

Signed-off-by: Yonghong Song <yhs@fb.com>