einfo.h, rc.h.in: ensure __BEGIN_DECLS is defined

Some Standard C Libraries, like musl, don't define __BEGIN_DECLS
or __END_DECLS.  We add some ifdef magic to ensure these are
available.

src/libeinfo/einfo.h

@@ -48,6 +48,16 @@
 # endif
 #endif
 
+#undef __BEGIN_DECLS
+#undef __END_DECLS
+#ifdef __cplusplus
+# define __BEGIN_DECLS extern "C" {
+# define __END_DECLS }
+#else
+# define __BEGIN_DECLS /* empty */
+# define __END_DECLS /* empty */
+#endif
+
 __BEGIN_DECLS
 
 /*! @brief Color types to use */