cmake: check for math library functions even in the absence of libm

(Reference issue: https://github.com/libsdl-org/SDL/issues/7011)

CMakeLists.txt

@@ -1047,6 +1047,7 @@ if(SDL_LIBC)
     check_library_exists(m pow "" HAVE_LIBM)
     if(HAVE_LIBM)
       set(CMAKE_REQUIRED_LIBRARIES m)
+    endif()
     foreach(_FN
               atan atan2 atanf atan2f ceil ceilf copysign copysignf cos cosf
               exp expf fabs fabsf floor floorf fmod fmodf log logf log10 log10f
@@ -1056,6 +1057,7 @@ if(SDL_LIBC)
         set(_HAVEVAR "HAVE_${_UPPER}")
         check_symbol_exists("${_FN}" "math.h" ${_HAVEVAR})
     endforeach()
+    if(HAVE_LIBM)
       set(CMAKE_REQUIRED_LIBRARIES)
       if(NOT VITA)
         list(APPEND EXTRA_LIBS m)