https://bugs.gentoo.org/910693
Fix implicit declarations by regenerating configure,
Fix config header templates, manually define _GNU_SOURCE  when
correct header was not cooperating
--- a/configure.ac
+++ b/configure.ac
@@ -29,6 +29,7 @@
 AC_CONFIG_AUX_DIR(config)
 AC_CONFIG_MACRO_DIR(m4)
 AC_LANG(C)
+AC_USE_SYSTEM_EXTENSIONS
 
 dnl Compute the canonical host (run-time) system type variable
 AC_CANONICAL_HOST
--- a/libtiff/tif_config.h.in
+++ b/libtiff/tif_config.h.in
@@ -139,6 +139,11 @@
 /* Support zstd compression */
 #undef ZSTD_SUPPORT
 
+/* Enable GNU extensions on systems that have them.  */
+#ifndef _GNU_SOURCE
+# undef _GNU_SOURCE
+#endif
+
 /* Enable large inode numbers on Mac OS X 10.5.  */
 #ifndef _DARWIN_USE_64_BIT_INODE
 # define _DARWIN_USE_64_BIT_INODE 1
--- a/libtiff/tiffconf.h.in
+++ b/libtiff/tiffconf.h.in
@@ -90,6 +90,11 @@
 /* Support Deflate compression */
 #undef ZIP_SUPPORT
 
+/* Enable GNU extensions on systems that have them.  */
+#ifndef _GNU_SOURCE
+# undef _GNU_SOURCE
+#endif
+
 /* Support libdeflate enhanced compression */
 #undef LIBDEFLATE_SUPPORT
 
--- a/libtiff/mkg3states.c
+++ b/libtiff/mkg3states.c
@@ -27,6 +27,7 @@
  * in Frank Cringle's viewfax program;
  *      Copyright (C) 1990, 1995  Frank D. Cringle.
  */
+#define _GNU_SOURCE 1
 #include "tif_config.h"
 #include "libport.h"