diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 65d1578..5a1e720 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -35,7 +35,7 @@ jobs:
       - name: Configure build
         run: |
           autoreconf --install
-          ./configure CFLAGS='-O0 -g -Wall -Wextra -Wpedantic'
+          ./configure CFLAGS='-O0 -g -std=c99 -Wall -Wextra -Wpedantic'
       - name: Compile
         run: make
       - name: Run tests
diff --git a/configure.ac b/configure.ac
index f34de68..12aacdf 100644
--- a/configure.ac
+++ b/configure.ac
@@ -49,5 +49,8 @@ m4_define([CTLSEQS_LT_REVISION], [0])
 m4_define([CTLSEQS_LT_AGE], [0])
 AC_SUBST([CTLSEQS_LT_VERSION], [CTLSEQS_LT_CURRENT:CTLSEQS_LT_REVISION:CTLSEQS_LT_AGE])
 
+# Assume that POSIX.1-2008 is supported.
+AC_DEFINE([_POSIX_C_SOURCE], [200809L], [Feature test macro for POSIX.1-2008])
+
 AC_CONFIG_FILES([Makefile doc/Makefile man/Makefile src/Makefile tests/Makefile examples/Makefile])
 AC_OUTPUT
diff --git a/examples/sixdraw.c b/examples/sixdraw.c
index 663d90e..1b39340 100644
--- a/examples/sixdraw.c
+++ b/examples/sixdraw.c
@@ -36,6 +36,7 @@
 #include <stdlib.h>
 
 #include <fcntl.h>
+#include <getopt.h>
 #include <sys/ioctl.h>
 #include <termios.h>
 #include <unistd.h>
diff --git a/tests/tcsgrep.c b/tests/tcsgrep.c
index 6fe340e..f26047d 100644
--- a/tests/tcsgrep.c
+++ b/tests/tcsgrep.c
@@ -28,6 +28,7 @@
 #include <stdlib.h>
 
 #include <fcntl.h>
+#include <getopt.h>
 #include <termios.h>
 #include <unistd.h>