Unified usage of the 'inline' attribute for functions

Autoconf provides the necessary stuff to detect if inline keyword
is supported, and to detect special syntaxes, so let's use this
and remove the multiple local definitions, this makes code simpler.

wrlib/misc.c

@@ -159,7 +159,7 @@ void RClearImage(RImage * image, const RColor * color)
 	}
 }
 
-static __inline__ unsigned char clip(int c)
+static inline unsigned char clip(int c)
 {
 	if (c > 255)
 		c = 255;