configure: Find correct libgsm's gsm.h header

Some distributions (archlinux) or versions of libgsm install gsm.h in
/usr/include/gsm/gsm.h

Since libgsm doesn't come with a pkfconfig, let's first check if gsm.h
and take the correct path in the build setup.

Change-Id: I07d3c03903e0d4bb80e843c7ed917a27b791ea53
diff --git a/include/osmocom/legacy_mgcp/mgcp_transcode.h b/include/osmocom/legacy_mgcp/mgcp_transcode.h
index 147e48b..6892deb 100644
--- a/include/osmocom/legacy_mgcp/mgcp_transcode.h
+++ b/include/osmocom/legacy_mgcp/mgcp_transcode.h
@@ -21,7 +21,11 @@
 
 #include "bscconfig.h"
 
+#if HAVE_GSM_H
 #include <gsm.h>
+#elif HAVE_GSM_GSM_H
+#include <gsm/gsm.h>
+#endif
 #ifdef HAVE_BCG729
 #include <bcg729/decoder.h>
 #include <bcg729/encoder.h>