This bug caused c_function_prototypes to not be recorded at configure
time when compiling with -std=gnu99 or similar. In the case of feature
recording, that was not a problem, because the logic in
CMakeDetermineCompileFeatures.cmake currently assumes that a feature
present for an earlier standard is present for a later standard.
However, the detection strings are also used in WriteCompilerDetectionHeader,
so the feature macro has been defined to '0' when using a later language
dialect.
Fix that by not checking the existence of the __STDC_VERSION__ macro at
all when detecting C90 features.
The __STDC_VERSION__ macro may be defined or not depending on the
implementation dialect of C. Test that it is defined before testing
its value.
The CXX tests do not need such a change because they define __cplusplus
in all dialects.
Stephen Kelly