cmake: Only add warning flags if the compiler supports them

I am compiling for FreeBSD where the compiler is Clang and doesn't accept
all the GCC warning flags. This breaks the -Werror build:
```
error: unknown warning option '-Wduplicated-branches' [-Werror,-Wunknown-warning-option]
error: unknown warning option '-Wduplicated-cond' [-Werror,-Wunknown-warning-option]
error: unknown warning option '-Wjump-misses-init' [-Werror,-Wunknown-warning-option]
error: unknown warning option '-Wlogical-op'; did you mean '-Wlong-long'? [-Werror,-Wunknown-warning-option]
error: unknown warning option '-Wrestrict' [-Werror,-Wunknown-warning-option]
error: unknown warning option '-Wunused-but-set-variable'; did you mean '-Wunused-const-variable'? [-Werror,-Wunknown-warning-option]
```

With this change we use check_{c,cxx}_compiler_flag to check if the flag
is supported before adding it. In the future this will allow adding
clang-specific warning flags to the list of warnings as well since they
will be ignored for GCC.

1 files changed, 1 insertions, 0 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index cc4926e3..890618dc 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -356,6 +356,7 @@ generate_compiler_warning_flags(
         ${WARNINGS_ERRORS}
 )
 generate_compiler_warning_flags(
+    CXX
     RESULTVAR
         WARNINGS_CXXFLAGS
     WARNINGS