Enable system extensions in hb.hh

Fixes https://github.com/harfbuzz/harfbuzz/issues/1491

configure.ac

@@ -12,7 +12,6 @@ AC_CONFIG_HEADERS([config.h])
 AM_INIT_AUTOMAKE([1.13.0 gnits tar-ustar dist-bzip2 no-dist-gzip -Wall no-define color-tests -Wno-portability])
 AM_SILENT_RULES([yes])
 AX_CODE_COVERAGE
-AC_USE_SYSTEM_EXTENSIONS
 
 # Initialize libtool
 m4_ifdef([AM_PROG_AR], [AM_PROG_AR])

src/hb.hh

@@ -33,6 +33,32 @@
 #include "config.h"
 #endif
 
+/*
+ * Following added based on what AC_USE_SYSTEM_EXTENSIONS adds to
+ * config.h.in.  Copied here for the convenience of those embedding
+ * HarfBuzz and not using our build system.
+ */
+/* Enable extensions on AIX 3, Interix.  */
+#ifndef _ALL_SOURCE
+# define _ALL_SOURCE 1
+#endif
+/* Enable GNU extensions on systems that have them.  */
+#ifndef _GNU_SOURCE
+# define _GNU_SOURCE 1
+#endif
+/* Enable threading extensions on Solaris.  */
+#ifndef _POSIX_PTHREAD_SEMANTICS
+# define _POSIX_PTHREAD_SEMANTICS 1
+#endif
+/* Enable extensions on HP NonStop.  */
+#ifndef _TANDEM_SOURCE
+# define _TANDEM_SOURCE 1
+#endif
+/* Enable general extensions on Solaris.  */
+#ifndef __EXTENSIONS__
+# define __EXTENSIONS__ 1
+#endif
+
 #ifndef _POSIX_C_SOURCE
 #define _POSIX_C_SOURCE 200809L
 #endif