Features: Fix C90 feature detection.

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.

Modules/Compiler/AppleClang-C-FeatureTests.cmake

@@ -7,5 +7,5 @@ set(AppleClang_C99 "${_cmake_oldestSupported} && defined(__STDC_VERSION__) && __
 set(_cmake_feature_test_c_restrict "${AppleClang_C99}")
 set(_cmake_feature_test_c_variadic_macros "${AppleClang_C99}")
 
-set(AppleClang_C90 "${_cmake_oldestSupported} && !defined(__STDC_VERSION__)")
+set(AppleClang_C90 "${_cmake_oldestSupported}")
 set(_cmake_feature_test_c_function_prototypes "${AppleClang_C90}")

Modules/Compiler/Clang-C-FeatureTests.cmake

@@ -7,5 +7,5 @@ set(Clang_C99 "${_cmake_oldestSupported} && defined(__STDC_VERSION__) && __STDC_
 set(_cmake_feature_test_c_restrict "${Clang_C99}")
 set(_cmake_feature_test_c_variadic_macros "${Clang_C99}")
 
-set(Clang_C90 "${_cmake_oldestSupported} && !defined(__STDC_VERSION__)")
+set(Clang_C90 "${_cmake_oldestSupported}")
 set(_cmake_feature_test_c_function_prototypes "${Clang_C90}")

Modules/Compiler/GNU-C-FeatureTests.cmake

@@ -13,5 +13,5 @@ set(GNU44_C99 "(__GNUC__ * 100 + __GNUC_MINOR__) >= 404 && defined(__STDC_VERSIO
 set(_cmake_feature_test_c_restrict "${GNU44_C99}")
 set(_cmake_feature_test_c_variadic_macros "${GNU44_C99}")
 
-set(GNU_C90 "${_cmake_oldestSupported} && !defined(__STDC_VERSION__)")
+set(GNU_C90 "${_cmake_oldestSupported}")
 set(_cmake_feature_test_c_function_prototypes "${GNU_C90}")