diff --git a/libs/freetdm/Makefile.am b/libs/freetdm/Makefile.am
index 2177a96ed1..bc92e1db58 100644
--- a/libs/freetdm/Makefile.am
+++ b/libs/freetdm/Makefile.am
@@ -49,6 +49,7 @@ if HAVE_SNG_ISDN
 INCS += -I/usr/include/sng_isdn
 endif 
 
+FTDM_COMPAT_CFLAGS = $(INCS) -DFTDM_CONFIG_DIR=\"@confdir@\" -DFTDM_MOD_DIR=\"$(moddir)\" @COMP_VENDOR_COMPAT_CFLAGS@ @DEFS@
 FTDM_CFLAGS = $(INCS) -DFTDM_CONFIG_DIR=\"@confdir@\" -DFTDM_MOD_DIR=\"$(moddir)\" @COMP_VENDOR_CFLAGS@ @DEFS@
 COMPILE   = $(CC) $(FTDM_CFLAGS)
 LTCOMPILE = $(LIBTOOL) --mode=compile --tag=CC $(COMPILE)
@@ -186,7 +187,9 @@ ftmod_analog_em_la_LIBADD  = libfreetdm.la
 if HAVE_LIBSANGOMA
 mod_LTLIBRARIES += ftmod_wanpipe.la
 ftmod_wanpipe_la_SOURCES = $(SRC)/ftmod/ftmod_wanpipe/ftmod_wanpipe.c
-ftmod_wanpipe_la_CFLAGS  = $(AM_CFLAGS) $(FTDM_CFLAGS) -D__LINUX__ -I/usr/include/wanpipe
+#some structures within Wanpipe drivers are not c99 compatible, so we need to compile ftmod_wanpipe
+#without c99 flags
+ftmod_wanpipe_la_CFLAGS  = $(AM_CFLAGS) $(FTDM_COMPAT_CFLAGS) -D__LINUX__ -I/usr/include/wanpipe
 ftmod_wanpipe_la_LDFLAGS = -shared -module -avoid-version -lsangoma
 ftmod_wanpipe_la_LIBADD  = libfreetdm.la
 endif
diff --git a/libs/freetdm/configure.ac b/libs/freetdm/configure.ac
index 2b5ded8e7d..8b878edc09 100644
--- a/libs/freetdm/configure.ac
+++ b/libs/freetdm/configure.ac
@@ -77,11 +77,14 @@ sun)
 	fi
 	;;
 *)
-	COMP_VENDOR_CFLAGS="-std=c99 -Wall -Wunused-variable -Wwrite-strings -Wstrict-prototypes -Wmissing-prototypes"
+	COMP_VENDOR_COMPAT_CFLAGS="-Wall -Wunused-variable -Wwrite-strings -Wstrict-prototypes -Wmissing-prototypes"
+	COMP_VENDOR_CFLAGS="-std=c99 $COMP_VENDOR_COMPAT_CFLAGS"
 	;;
 esac
+AC_SUBST([COMP_VENDOR_COMPAT_CFLAGS])
 AC_SUBST([COMP_VENDOR_CFLAGS])
 
+
 #  Enable debugging
 AC_ARG_ENABLE([debug],
 	[AC_HELP_STRING([--enable-debug], [build with debug information])],