libgsm: detect libgsm header path

Libgsm header can reside either in the base include dir or in
the gsm subdir.
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

configure

@@ -1235,6 +1235,7 @@ HAVE_LIST="
     getservbyport
     gettimeofday
     gnu_as
+    gsm_h
     ibm_asm
     inet_aton
     io_h
@@ -3474,7 +3475,9 @@ enabled gnutls     && require_pkg_config gnutls gnutls/gnutls.h gnutls_global_in
 enabled libfaac    && require2 libfaac "stdint.h faac.h" faacEncGetVersion -lfaac
 enabled libfdk_aac && require  libfdk_aac fdk-aac/aacenc_lib.h aacEncOpen -lfdk-aac
 enabled libfreetype && require_pkg_config freetype2 "ft2build.h freetype/freetype.h" FT_Init_FreeType
-enabled libgsm     && require  libgsm gsm/gsm.h gsm_create -lgsm
+enabled libgsm     && { for gsm_hdr in "gsm.h" "gsm/gsm.h"; do
+                            check_lib "${gsm_hdr}" gsm_create -lgsm && break;
+                        done || die "ERROR: libgsm not found"; }
 enabled libilbc    && require  libilbc ilbc.h WebRtcIlbcfix_InitDecode -lilbc
 enabled libmp3lame && require  "libmp3lame >= 3.98.3" lame/lame.h lame_set_VBR_quality -lmp3lame
 enabled libopencore_amrnb  && require libopencore_amrnb opencore-amrnb/interf_dec.h Decoder_Interface_init -lopencore-amrnb

libavcodec/libgsm.c

@@ -27,7 +27,12 @@
 
 // The idiosyncrasies of GSM-in-WAV are explained at http://kbs.cs.tu-berlin.de/~jutta/toast.html
 
+#include "config.h"
+#if HAVE_GSM_H
+#include <gsm.h>
+#else
 #include <gsm/gsm.h>
+#endif
 
 #include "libavutil/channel_layout.h"
 #include "libavutil/common.h"