use macros instead of inline functions in shgetc.h

at -Os optimization level, gcc refuses to inline these functions even
though the inlined code would roughly the same size as the function
call, and much faster. the easy solution is to make them into macros.

src/internal/shgetc.h

@@ -3,23 +3,7 @@
 void __shlim(FILE *, off_t);
 int __shgetc(FILE *);
 
-static inline off_t shcnt(FILE *f)
-{
-	return f->shcnt + (f->rpos - f->rend);
-}
-
-static inline void shlim(FILE *f, off_t lim)
-{
-	__shlim(f, lim);
-}
-
-static inline int shgetc(FILE *f)
-{
-	if (f->rpos < f->shend) return *f->rpos++;
-	return __shgetc(f);
-}
-
-static inline void shunget(FILE *f)
-{
-	if (f->rend) f->rpos--;
-}
+#define shcnt(f) ((f)->shcnt + ((f)->rpos - (f)->rend))
+#define shlim(f, lim) __shlim((f), (lim))
+#define shgetc(f) (((f)->rpos < (f)->shend) ? *(f)->rpos++ : __shgetc(f))
+#define shunget(f) ((f)->rend ? (void)(f)->rpos-- : (void)0)