Avoid redefining macros defined by the system

In particular, OpenBSD already defines __GNUC_PREREQ__, __BEGIN_DECLS,
and __warn_references. Hiding the definitions behind #ifndef guards
avoids warnings when building.

src/bsd_cdefs.h

@@ -78,12 +78,14 @@
 /*
  * Macro to test if we're using a specific version of gcc or later.
  */
+#ifndef __GNUC_PREREQ__
 #if defined(__GNUC__) && !defined(__INTEL_COMPILER)
 #define	__GNUC_PREREQ__(ma, mi)	\
 	(__GNUC__ > (ma) || __GNUC__ == (ma) && __GNUC_MINOR__ >= (mi))
 #else
 #define	__GNUC_PREREQ__(ma, mi)	0
 #endif
+#endif /* __GNUC_PREREQ__ */
 
 /*
  * Compiler-dependent macro to help declare pure (no side effects) functions.

src/cdefs-compat.h

@@ -1,6 +1,7 @@
 #ifndef _CDEFS_COMPAT_H_
 #define	_CDEFS_COMPAT_H_
 
+#if !defined(__BEGIN_DECLS)
 #if defined(__cplusplus)
 #define	__BEGIN_DECLS	extern "C" {
 #define	__END_DECLS	}
@@ -8,6 +9,7 @@
 #define	__BEGIN_DECLS
 #define	__END_DECLS
 #endif
+#endif /* !defined(__BEGIN_DECLS) */
 
 #ifdef __GNUC__
 #ifndef __strong_reference
@@ -25,18 +27,22 @@
 #define	__weak_reference(sym,alias)	\
 	__asm__(".weak " #alias);	\
 	__asm__(".equ "  #alias ", " #sym)
+#ifndef __warn_references
 #define	__warn_references(sym,msg)	\
 	__asm__(".section .gnu.warning." #sym);	\
 	__asm__(".asciz \"" msg "\"");	\
 	__asm__(".previous")
+#endif /* __warn_references */
 #else
 #define	__weak_reference(sym,alias)	\
 	__asm__(".weak alias");		\
 	__asm__(".equ alias, sym")
+#ifndef __warn_references
 #define	__warn_references(sym,msg)	\
 	__asm__(".section .gnu.warning.sym"); \
 	__asm__(".asciz \"msg\"");	\
 	__asm__(".previous")
+#endif	/* __warn_references */
 #endif	/* __STDC__ */
 #elif defined(__clang__) /* CLANG */
 #ifdef __STDC__
@@ -53,16 +59,20 @@
 #define __weak_reference(sym,alias)	\
 	__asm__(".stabs \"_" #alias "\",11,0,0,0");	\
 	__asm__(".stabs \"_" #sym "\",1,0,0,0")
+#ifndef __warn_references
 #define __warn_references(sym,msg)	\
 	__asm__(".stabs \"" msg "\",30,0,0,0");		\
 	__asm__(".stabs \"_" #sym "\",1,0,0,0")
+#endif /* __warn_references */
 #else
 #define __weak_reference(sym,alias)	\
 	__asm__(".stabs \"_/**/alias\",11,0,0,0");	\
 	__asm__(".stabs \"_/**/sym\",1,0,0,0")
+#ifndef __warn_references
 #define __warn_references(sym,msg)	\
 	__asm__(".stabs msg,30,0,0,0");			\
 	__asm__(".stabs \"_/**/sym\",1,0,0,0")
+#endif	/* __warn_references */
 #endif	/* __STDC__ */
 #endif	/* __ELF__ */
 #endif  /* __weak_reference */