https://bugs.gentoo.org/898866
From: Brahmajit Das <brahmajit.xyz@gmail.com>
Date: Sat, 17 Jun 2023 15:32:10 +0000
Subject: [PATCH 1/1] Fix building against musl C library

PTHREAD_RWLOCK_PREFER_WRITER_NP isn't available in musl libc/non-glibc
systems. Additional check of __GLIBC__ being defined is needed.

Signed-off-by: Brahmajit Das <brahmajit.xyz@gmail.com>
--- a/lib/FXReadWriteLock.cpp
+++ b/lib/FXReadWriteLock.cpp
@@ -62,7 +62,7 @@ FXReadWriteLock::FXReadWriteLock(){
   InitializeCriticalSection(((RWLOCK*)data)->mutex);
   InitializeCriticalSection(((RWLOCK*)data)->access);
   ((RWLOCK*)data)->readers=0;
-#elif (_XOPEN_SOURCE >= 500) || (_POSIX_C_SOURCE >= 200809L)
+#elif (_XOPEN_SOURCE >= 500) || (_POSIX_C_SOURCE >= 200809L) && defined(__GLIBC__)
   // If this fails on your machine, determine what value
   // of sizeof(pthread_rwlock_t) is supposed to be on your
   // machine and mail it to: jeroen@fox-toolkit.net!!