Don't set _POSIX_C_SOURCE in NetBSD
According to a harfbuzz package patch on NetBSD project
https://github.com/NetBSD/pkgsrc/blob/trunk/fonts/harfbuzz/patches/patch-src_hb-blob.cc
diff --git a/src/hb-blob.cc b/src/hb-blob.cc
index 699f66b..5783fb2 100644
--- a/src/hb-blob.cc
+++ b/src/hb-blob.cc
@@ -30,7 +30,7 @@
* http://www.gnu.org/software/libc/manual/html_node/Feature-Test-Macros.html
* https://www.oracle.com/technetwork/articles/servers-storage-dev/standardheaderfiles-453865.html
*/
-#if !defined(_POSIX_C_SOURCE) && !defined(_MSC_VER)
+#if !defined(_POSIX_C_SOURCE) && !defined(_MSC_VER) && !defined(__NetBSD__)
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-macros"
#define _POSIX_C_SOURCE 200809L
