diff --git a/configure b/configure
index ad5325a..ea7fe2c 100755
--- a/configure
+++ b/configure
@@ -2,6 +2,7 @@
 
 prefix=/usr/local
 OUR_CPPFLAGS=
+C99_CFLAGS="-std=c99"
 
 # Get a temporary filename
 i=0
@@ -42,7 +43,7 @@ check_compile() {
 	printf "checking %s ... " "$1"
 	printf "$3" > "$tmpc"
 	local res=0
-	$CC $OUR_CPPFLAGS $CPPFLAGS $2 $CFLAGS -c "$tmpc" -o /dev/null >/dev/null 2>&1 \
+	$CC $OUR_CPPFLAGS $CPPFLAGS $2 $C99_CFLAGS $CFLAGS -c "$tmpc" -o /dev/null >/dev/null 2>&1 \
 	|| res=1
 	test x$res = x0 && \
 	{ printf "yes\n" ; test x"$2" = x || OUR_CPPFLAGS="$OUR_CPPFLAGS $2" ; } \
@@ -53,7 +54,7 @@ check_compile() {
 check_define() {
 	printf "checking whether \$CC defines %s ... " "$1"
 	local res=1
-	$CC $OUR_CPPFLAGS $CPPFLAGS $CFLAGS -dM -E - </dev/null | grep "$1" >/dev/null && res=0
+	$CC $OUR_CPPFLAGS $CPPFLAGS $C99_CFLAGS $CFLAGS -dM -E - </dev/null | grep "$1" >/dev/null && res=0
 	test x$res = x0 && printf "yes\n" || printf "no\n"
 	return $res
 }
@@ -62,7 +63,7 @@ check_compile_run() {
 	printf "checking %s ... " "$1"
 	printf "$2" > "$tmpc"
 	local res=0
-	$CC $OUR_CPPFLAGS $CPPFLAGS $CFLAGS "$tmpc" -o "$tmpc".out >/dev/null 2>&1 \
+	$CC $OUR_CPPFLAGS $CPPFLAGS $C99_CFLAGS $CFLAGS "$tmpc" -o "$tmpc".out >/dev/null 2>&1 \
 	|| res=1
 	test x$res = x0 && { "$tmpc".out || res=1 ; }
 	rm -f "$tmpc".out
@@ -141,6 +142,10 @@ if [ -z "$CC" ] ; then
 	CC=cc
 fi
 
+for i in 700 600; do
+	check_compile "whether _XOPEN_SOURCE=$i is suitable for C99 mode" "-D_XOPEN_SOURCE=$i" "#include <unistd.h>" && break
+done
+
 for i in 16 32; do
 	check_compile "whether netinet/in.h defines s6_addr$i" "" \
 	'#include <netinet/in.h>\nint main(int a, char**c){'"struct in6_addr x={.s6_addr$i[0]=a};return x.s6_addr$i[0];"' }' \
diff --git a/src/allocator_thread.c b/src/allocator_thread.c
index 314f926..cb4e987 100644
--- a/src/allocator_thread.c
+++ b/src/allocator_thread.c
@@ -3,7 +3,6 @@
 #undef _POSIX_C_SOURCE
 #define _POSIX_C_SOURCE 200809L
 #define _DARWIN_C_SOURCE
-#define _XOPEN_SOURCE 600
 #include <limits.h>
 #include <pthread.h>
 #include <unistd.h>
diff --git a/src/main.c b/src/main.c
index eb7465f..1f33003 100644
--- a/src/main.c
+++ b/src/main.c
@@ -9,10 +9,7 @@
 
 #undef _POSIX_C_SOURCE
 #define _POSIX_C_SOURCE 200809L
-#undef _XOPEN_SOURCE
-#ifdef IS_SOLARIS	/* Solaris doesn't recognize this macro greater than 600 */
-#define _XOPEN_SOURCE 600
-#else
+#ifndef _XOPEN_SOURCE
 #define _XOPEN_SOURCE 700
 #endif
 #include <stdio.h>