utils: Move thread-safe strerror replacement to a separate file

For some utils _GNU_SOURCE might be needed but that conflicts with the
signature of strerror_r(3).

src/libstrongswan/utils/utils.h

@@ -30,6 +30,7 @@
 #include <string.h>
 
 #include "enum.h"
+#include "utils/strerror.h"
 
 /**
  * strongSwan program return codes
@@ -494,23 +495,6 @@ char *strreplace(const char *str, const char *search, const char *replace);
  */
 bool mkdir_p(const char *path, mode_t mode);
 
-/**
- * Thread-safe wrapper around strerror and strerror_r.
- *
- * This is required because the first is not thread-safe (on some platforms)
- * and the second uses two different signatures (POSIX/GNU) and is impractical
- * to use anyway.
- *
- * @param errnum	error code (i.e. errno)
- * @return			error message
- */
-const char *safe_strerror(int errnum);
-
-/**
- * Replace usages of strerror(3) with thread-safe variant.
- */
-#define strerror(errnum) safe_strerror(errnum)
-
 #ifndef HAVE_CLOSEFROM
 /**
  * Close open file descriptors greater than or equal to lowfd.