config, os: add support for Musl libc

According to Musl's FAQ, `__BEGIN_DECLS` and `__END_DECLS` macros come from Glibc private headers. Because of that, we need to define them manually in case those aren't defined.

Additionally, for Musl we need to override `HAVE_DECL_PROGRAM_INVOCATION_SHORT_NAME`, for which a corresponding `#ifndef` was added in `config.h`.

Author: MaxDesiatov

config/config.h

@@ -47,7 +47,9 @@
 
 /* Define to 1 if you have the declaration of `program_invocation_short_name',
    and to 0 if you don't. */
+#ifndef HAVE_DECL_PROGRAM_INVOCATION_SHORT_NAME
 #define HAVE_DECL_PROGRAM_INVOCATION_SHORT_NAME 0
+#endif
 
 /* Define to 1 if you have the declaration of `SIGEMT', and to 0 if you don't.
    */

os/generic_win_base.h

@@ -25,14 +25,6 @@
 typedef int mode_t;
 typedef void pthread_attr_t;
 
-#if defined(__cplusplus)
-#define __BEGIN_DECLS extern "C" {
-#define __END_DECLS }
-#else
-#define __BEGIN_DECLS
-#define __END_DECLS
-#endif
-
 #ifndef API_AVAILABLE
 #define API_AVAILABLE(...)
 #endif

os/object.h

@@ -21,6 +21,16 @@
 #ifndef __OS_OBJECT__
 #define __OS_OBJECT__
 
+#if !defined(__BEGIN_DECLS) && !defined(__END_DECLS)
+#if defined(__cplusplus)
+#define __BEGIN_DECLS extern "C" {
+#define __END_DECLS }
+#else
+#define __BEGIN_DECLS
+#define __END_DECLS
+#endif
+#endif
+
 #ifdef __APPLE__
 #include <Availability.h>
 #include <os/availability.h>