library: replace the troublesome '__BEGIN_DECLS' macro

When 'newlib' was introduced, in the commit referenced
below, the use of that glibc '__BEGIN_DECLS' macro was
standardized. However, as issue #88 revealed, this may
result in a fatal build error with other environments.

So, this patch just trades that macro for the standard
'#ifdef __cplusplus' conventions (thus avoiding use of
all those '#include <features.h>' directives as well).

Reference(s):
. newlib introduced
commit a410e236ab
. procps-ng-3.3.13 issue
https://gitlab.com/procps-ng/procps/issues/88
. some additional discussion
https://www.freelists.org/post/procps/PATCH-Replace-glibcspecific-macros-in-procnumah,1
. musl wiki (see: sys/cdefs.h error messages)
https://wiki.musl-libc.org/faq.html

Signed-off-by: Jim Warner <james.warner@comcast.net>

proc/namespace.h

@@ -23,7 +23,9 @@
 #ifndef PROC_NAMESPACE_H
 #define PROC_NAMESPACE_H
 
-__BEGIN_DECLS
+#ifdef __cplusplus
+extern "C" {
+#endif
 
 enum namespace_type {
     PROCPS_NS_IPC,
@@ -44,6 +46,8 @@ const char *procps_ns_get_name(const int id);
 int procps_ns_get_id(const char *name);
 
 int procps_ns_read_pid(const int pid, struct procps_namespaces *nsp);
-__END_DECLS
 
+#ifdef __cplusplus
+}
+#endif
 #endif