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

configure.ac

@@ -80,6 +80,13 @@ AC_ARG_ENABLE([mgcp-transcoding], [AS_HELP_STRING([--enable-mgcp-transcoding], [
 AC_ARG_WITH([g729], [AS_HELP_STRING([--with-g729], [Enable G.729 encoding/decoding.])], [osmo_ac_with_g729="$withval"],[osmo_ac_with_g729="no"])
 
 if test "$osmo_ac_mgcp_transcoding" = "yes" ; then
+    AC_CHECK_HEADERS([gsm.h gsm/gsm.h], [osmo_ac_found_gsm_headers=yes])
+    if test "$osmo_ac_found_gsm_headers" != "yes" ; then
+	AC_MSG_ERROR([Unable to find the libgsm headers])
+    fi
+    AC_SUBST(HAVE_GSM_H)
+    AC_SUBST(HAVE_GSM_GSM_H)
+
     AC_SEARCH_LIBS([gsm_create], [gsm], [LIBRARY_GSM="$LIBS";LIBS=""], [AC_MSG_ERROR([--enable-mgcp-transcoding: cannot find usable libgsm])])
     AC_SUBST(LIBRARY_GSM)
     if test "$osmo_ac_with_g729" = "yes" ; then

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>