The syncfs probe in ports/unix/subports fails on glibc systems if the
compiler does not support implicit function declarations. Defining
_GNU_SOURCE makes sure that <unistd.h> provides a function prototype
for syncfs.

Submitted upstream: <https://bugzilla.yoctoproject.org/show_bug.cgi?id=15074>

diff -ur pseudo-1.9.0.orig/ports/unix/subports pseudo-1.9.0/ports/unix/subports
--- pseudo-1.9.0.orig/ports/unix/subports	2018-01-23 21:18:03.000000000 +0100
+++ pseudo-1.9.0/ports/unix/subports	2023-03-17 08:52:55.179082920 +0100
@@ -1,5 +1,6 @@
 #!/bin/sh
 cat > dummy.c <<EOF
+#define _GNU_SOURCE
 #include <unistd.h>
 int main(void) {
 	syncfs(0);