Add and update include guards

Include guards in most changed files were missing, I added them keeping
the same style as other SDL files. In some cases I moved the include
guards around to be the first thing the header has to take advantage of
any possible improvements compiler may have for inclusion guards.

src/libm/math_libm.h

@@ -18,6 +18,10 @@
      misrepresented as being the original software.
   3. This notice may not be removed or altered from any source distribution.
 */
+
+#ifndef math_libm_h_
+#define math_libm_h_
+
 #include "../SDL_internal.h"
 
 /* Math routines from uClibc: http://www.uclibc.org */
@@ -38,4 +42,6 @@ double SDL_uclibc_sin(double x);
 double SDL_uclibc_sqrt(double x);
 double SDL_uclibc_tan(double x);
 
+#endif /* math_libm_h_ */
+
 /* vi: set ts=4 sw=4 expandtab: */