Don't use -fvisibility=hidden on Windows, since MinGW doesn't support it. :(

CMakeLists.txt

@@ -67,9 +67,10 @@ IF(CMAKE_COMPILER_IS_GNUCC)
     " PHYSFS_IS_GCC4)
 
     IF(PHYSFS_IS_GCC4)
-        IF(NOT OS2 AND NOT SOLARIS) # Not supported on OS/2 or Solaris.
+        # Not supported on several operating systems at this time.
+        IF(NOT OS2 AND NOT SOLARIS AND NOT WINDOWS) 
             ADD_DEFINITIONS(-fvisibility=hidden)
-        ENDIF(NOT OS2 AND NOT SOLARIS)
+        ENDIF(NOT OS2 AND NOT SOLARIS AND NOT WINDOWS)
     ENDIF(PHYSFS_IS_GCC4)
 ENDIF(CMAKE_COMPILER_IS_GNUCC)