request ortp >=0.15.0

build fails with ortp <0.15.0 because of incompatible API.
diff --git a/configure.ac b/configure.ac
index ec5086d..1ab807b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -37,7 +37,7 @@
 PKG_CHECK_MODULES(LIBOSMOCORE, libosmocore >= 0.3.0)
 PKG_CHECK_MODULES(LIBOSMOVTY, libosmovty >= 0.3.0)
 PKG_CHECK_MODULES(LIBOSMOGSM, libosmogsm >= 0.3.0)
-PKG_CHECK_MODULES(ORTP, ortp)
+PKG_CHECK_MODULES(ORTP, ortp >= 0.15.0)
 
 AC_CHECK_HEADERS(dahdi/user.h,,AC_MSG_WARN(DAHDI input driver will not be built))