Fixed compiler warnings with SDL_FORCE_INLINE on gcc2. (Thanks, Axel!)

 Fixes Bugzilla #1770.
Don't use __builtin_clz() on gcc2. (thanks, Axel!)

 Fixes Bugzilla #1771.
Patched test apps to compile with C89 compilers (thanks, Axel!).

 Fixes Bugzilla #1772.
Haiku no longer #defines __BEOS__, so check for it too (thanks, Axel!).

 Fixes Bugzilla #1773.
Corrected function signature on SDL_strtod_inline() (thanks, Axel!).

 Fixes Bugzilla #1774.