Quiet compiler warnings about missing prototypes in Linux's bitops.h.

src/backend/port/linux/asm/bitops.h

+/* This is a hack that lets us use the -Wmissing-prototypes compile option.
+
+   A bug or weakness in Linux's asm/bitops.h file makes it define a bunch
+   of inline functions without first declaring a prototype.  This causes
+   -Wmissing-prototypes to generate warnings.  These warnings are distracting
+   and, in the case of -Werror, fatal.
+
+   asm/bitops.h gets included by the Linux C library sem.h, which is included
+   in several Postgres backend source files.
+
+   Until Linux is fixed, we have our own version of asm/bitops.h that gets
+   included first because it is in a directory mentioned in a -I option,
+   whereas the Linux asm/bitops.h is in a standard include directory.  (This
+   is GNU C preprocessor function).
+
+   Our asm/bitops.h declares prototypes and then includes the Linux 
+   asm/bitops.h.  If Linux changes these functions, our asm/bitops.h will
+   stop compiling and will have to be updated.
+
+   -Bryan 1996.11.17
+*/
+
+#ifndef POSTGRES_BITOPS_H
+#define POSTGRES_BITOPS_H
+
+#ifdef __SMP__
+#define PG_BITOPS_VOLATILE volatile
+#else
+#define PG_BITOPS_VOLATILE 
+#endif
+
+extern __inline__ int set_bit(int nr, PG_BITOPS_VOLATILE void * addr);
+extern __inline__ int clear_bit(int nr, PG_BITOPS_VOLATILE void * addr);
+extern __inline__ int change_bit(int nr, PG_BITOPS_VOLATILE void * addr);
+extern __inline__ int test_bit(int nr, const PG_BITOPS_VOLATILE void * addr);
+extern __inline__ int find_first_zero_bit(void * addr, unsigned size);
+extern __inline__ int find_next_zero_bit (void * addr, int size, int offset);
+extern __inline__ unsigned long ffz(unsigned long word);
+
+#include_next <asm/bitops.h>
+#endif