diff --git a/configure.ac b/configure.ac
index f8c5b4f79..ea0abbb1d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1577,8 +1577,13 @@ if test "$GCC" = yes; then
     fi
 
     CFLAGS="$mycflags $mycflags_save"
+    USE_C99_CFLAGS="-std=gnu99"
+else
+    USE_C99_CFLAGS=
 fi
 
+AC_SUBST(USE_C99_CFLAGS)
+
 
 #
 # This is handy for debugging so the compiler doesn't rearrange
diff --git a/dirmngr/Makefile.am b/dirmngr/Makefile.am
index 5c294688d..f18786b8d 100644
--- a/dirmngr/Makefile.am
+++ b/dirmngr/Makefile.am
@@ -35,7 +35,8 @@ AM_CPPFLAGS = -I$(top_srcdir)/common
 
 include $(top_srcdir)/am/cmacros.am
 
-AM_CFLAGS = $(LIBGCRYPT_CFLAGS) $(KSBA_CFLAGS) $(LIBASSUAN_CFLAGS)	\
+AM_CFLAGS = $(USE_C99_CFLAGS) \
+            $(LIBGCRYPT_CFLAGS) $(KSBA_CFLAGS) $(LIBASSUAN_CFLAGS)	\
             $(GPG_ERROR_CFLAGS) $(NPTH_CFLAGS) $(NTBTLS_CFLAGS)		\
             $(LIBGNUTLS_CFLAGS)
 
@@ -132,7 +133,7 @@ endif
 
 # http tests
 t_http_SOURCES = $(t_common_src) t-http.c http.c dns-stuff.c
-t_http_CFLAGS  = -DWITHOUT_NPTH=1 \
+t_http_CFLAGS  = -DWITHOUT_NPTH=1  $(USE_C99_CFLAGS) \
 	         $(LIBGCRYPT_CFLAGS) $(NTBTLS_CFLAGS) $(LIBGNUTLS_CFLAGS) \
                  $(GPG_ERROR_CFLAGS)
 t_http_LDADD   = $(t_common_ldadd) \
@@ -142,11 +143,11 @@ t_ldap_parse_uri_SOURCES = \
 	t-ldap-parse-uri.c ldap-parse-uri.c ldap-parse-uri.h \
         http.c dns-stuff.c \
         $(ldap_url) $(t_common_src)
-t_ldap_parse_uri_CFLAGS = -DWITHOUT_NPTH=1 \
+t_ldap_parse_uri_CFLAGS = -DWITHOUT_NPTH=1  $(USE_C99_CFLAGS) \
 			  $(LIBGCRYPT_CFLAGS) $(GPG_ERROR_CFLAGS)
 t_ldap_parse_uri_LDADD = $(ldaplibs) $(t_common_ldadd) $(DNSLIBS)
 
-t_dns_stuff_CFLAGS = -DWITHOUT_NPTH=1 \
+t_dns_stuff_CFLAGS = -DWITHOUT_NPTH=1  $(USE_C99_CFLAGS) \
 		     $(LIBGCRYPT_CFLAGS) $(GPG_ERROR_CFLAGS)
 t_dns_stuff_SOURCES = $(t_common_src) t-dns-stuff.c dns-stuff.c
 t_dns_stuff_LDADD   = $(t_common_ldadd) $(DNSLIBS)