fix warning from -Wundef

Author: biojppm

.github/workflows/ubuntu24.yml

@@ -11,7 +11,7 @@ jobs:
         run: |
           mkdir build &&
           cd build &&
-          CXXFLAGS=-Werror cmake -DFASTFLOAT_TEST=ON -D FASTFLOAT_BENCHMARKS=ON ..  &&
+          "CXXFLAGS=-Werror -Wundef" cmake -DFASTFLOAT_TEST=ON -D FASTFLOAT_BENCHMARKS=ON ..  &&
           cmake --build .   &&
           ctest --output-on-failure
       - name: Use cmake CXX23

include/fast_float/float_common.h

@@ -9,7 +9,7 @@
 #include <type_traits>
 #include <system_error>
 #ifdef __has_include
-#if __has_include(<stdfloat>) && (__cplusplus > 202002L || _MSVC_LANG > 202002L)
+#if __has_include(<stdfloat>) && (__cplusplus > 202002L || (defined(_MSVC_LANG) && (_MSVC_LANG > 202002L)))
 #include <stdfloat>
 #endif
 #endif