cmake: Test each compiler flag
cmake: Test each compiler flag

Add non-trival C compiler flags only if check_c_compiler_flag()
succeeds.

This fixes the build with 4.8.1, which was broken by

 commit cbab09dc0e6eb8fcea40db92319305d5e136d4b9
 cmake: Compile with stricter error checking

 Add -Werror=incompatible-pointer-types and -Werror=int-conversion
 to the compiler flags.

Signed-off-by: Chad Versace <a557f7f75bf28dc19ac99107f5cc7cd6b3fbdb1a@linux.intel.com>

evelikov/waffle,fjhenigman/waffle,dcbaker/waffle,yinquan529/waffle-gl-waffle,maurossi/waffle,janesma/waffle,yinquan529/waffle-gl-waffle,dcbaker/waffle,waffle-gl/waffle,fjhenigman/waffle,evelikov/waffle,janesma/waffle,dcbaker/waffle,waffle-gl/waffle,chadversary/waffle,fjhenigman/waffle,janesma/waffle,maurossi/waffle,maurossi/waffle,waffle-gl/waffle,chadversary/waffle,evelikov/waffle,yinquan529/waffle-gl-waffle,chadversary/waffle,waffle-gl/waffle