build: Add a new libbsd_strong_alias() macro and switch users to it

We had several cases of code needing a strong alias, so we switch those
to use the new macro. This covers systems that support the alias
attribute and others such as macOS where we need to use assembler
directives to add the alias as the attribute is not supported.

src/local-link.h

@@ -37,6 +37,16 @@
 #define libbsd_link_warning(symbol, msg)
 #endif
 
+#if defined(__APPLE__)
+#define libbsd_strong_alias(alias, symbol) \
+	__asm__(".globl _" #alias); \
+	__asm__(".set _" #alias ", _" #symbol); \
+	extern __typeof(symbol) alias
+#elif !defined(_MSC_VER)
+#define libbsd_strong_alias(alias, symbol) \
+	extern __typeof__(symbol) alias __attribute__((__alias__(#symbol)))
+#endif
+
 #ifdef __ELF__
 #  if __has_attribute(symver)
 /* The symver attribute is supported since gcc 10.x. */
@@ -64,7 +74,7 @@
 #  endif
 #else
 #define libbsd_symver_default(alias, symbol, version) \
-	extern __typeof__(symbol) alias __attribute__((__alias__(#symbol)))
+	libbsd_strong_alias(alias, symbol)
 
 #define libbsd_symver_variant(alias, symbol, version)
 

src/setproctitle.c

@@ -295,10 +295,8 @@ libbsd_symver_default(setproctitle, setproctitle_impl, LIBBSD_0.5);
  * in 0.5, make the implementation available in the old version as an alias
  * for code linking against that version, and change the default to use the
  * new version, so that new code depends on the implemented version. */
-#ifdef HAVE_TYPEOF
-extern __typeof__(setproctitle_impl)
-setproctitle_stub
-	__attribute__((__alias__("setproctitle_impl")));
+#if defined(libbsd_strong_alias)
+libbsd_strong_alias(setproctitle_stub, setproctitle_impl);
 #else
 void
 setproctitle_stub(const char *fmt, ...)