Fix up physfs.h for compilers that are sensitive about preprocessor defines.

(transplanted from db8f944df5c56f0244229813203fb5b24e8d9968)

src/physfs.h

@@ -225,11 +225,11 @@ extern "C" {
 
 #if defined(PHYSFS_DECL)
 /* do nothing. */
-#elif (defined _MSC_VER)
+#elif defined(_MSC_VER)
 #define PHYSFS_DECL __declspec(dllexport)
-#elif (defined __SUNPRO_C)
+#elif defined(__SUNPRO_C)
 #define PHYSFS_DECL __global
-#elif ((__GNUC__ >= 3) && (!__EMX__) && (!sun))
+#elif ((__GNUC__ >= 3) && (!defined(__EMX__)) && (!defined(sun)))
 #define PHYSFS_DECL __attribute__((visibility("default")))
 #else
 #define PHYSFS_DECL