diff --git a/src/aclocal.m4 b/src/aclocal.m4
index 24e38999f..84da6bfa8 100644
--- a/src/aclocal.m4
+++ b/src/aclocal.m4
@@ -283,7 +283,9 @@ case "${host_os}" in
 		ECL_LDRPATH='-Wl,-R,~A'
 		TCPLIBS='-lsocket -lnsl -lintl'
 		clibs='-ldl'
-		CFLAGS="${CFLAGS} -std=gnu99"
+		# We should use C99 and _XOPEN_SOURCE=600, but Solaris 10
+		# ships with GCC 3.4.3 which does not support C99
+		# CFLAGS="${CFLAGS} -std=gnu99"
 		enable_slow_config=yes
 		;;
 	cygwin*)
diff --git a/src/c/package.d b/src/c/package.d
index 9c2d632ee..6eb387622 100644
--- a/src/c/package.d
+++ b/src/c/package.d
@@ -15,7 +15,9 @@
     See file '../Copyright' for full details.
 */
 
+#ifndef __sun__ /* See unixint.d for the reason of this */
 #define _XOPEN_SOURCE 600	/* For pthread mutex attributes */
+#endif
 #include <ecl/ecl.h>
 #include <ecl/internal.h>
 #include <ecl/ecl-inl.h>
diff --git a/src/c/unixint.d b/src/c/unixint.d
index d8d66a06f..6b654c242 100644
--- a/src/c/unixint.d
+++ b/src/c/unixint.d
@@ -15,7 +15,12 @@
     See file '../Copyright' for full details.
 */
 
-#define _XOPEN_SOURCE 600 /* For SA_SIGINFO in Solaris */
+#ifdef __sun__
+/* For SA_SIGINFO in Solaris. We could have used _XOPEN_SOURCE=600, but
+ * this requires C99 and the default GCC for Solaris (3.4.3) does not
+ * support this C standard. */
+#define __EXTENSIONS__
+#endif
 #include <errno.h>
 #include <string.h>
 #include <stdio.h>
diff --git a/src/configure b/src/configure
index 9e1b118e3..472312ef7 100755
--- a/src/configure
+++ b/src/configure
@@ -4432,7 +4432,9 @@ case "${host_os}" in
 		ECL_LDRPATH='-Wl,-R,~A'
 		TCPLIBS='-lsocket -lnsl -lintl'
 		clibs='-ldl'
-		CFLAGS="${CFLAGS} -std=gnu99"
+		# We should use C99 and _XOPEN_SOURCE=600, but Solaris 10
+		# ships with GCC 3.4.3 which does not support C99
+		# CFLAGS="${CFLAGS} -std=gnu99"
 		enable_slow_config=yes
 		;;
 	cygwin*)