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.

Author: MaxDesiatov

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