build: Remove check for gsm/gsm.h for libgsm

libgsm never installed more than a single header, the headers installed into
the gsm/ subdirectory by some distros are private. Who started this nonsense
is a mystery, but it got cargo-culted around ever since.

configure

@@ -1569,7 +1569,6 @@ HEADERS_LIST="
     direct_h
     dxgidebug_h
     dxva_h
-    gsm_h
     io_h
     mach_mach_time_h
     machine_ioctl_bt848_h
@@ -4756,9 +4755,7 @@ enabled libfaac           && require libfaac "stdint.h faac.h" faacEncGetVersion
 enabled libfdk_aac        && require_pkg_config libfdk_aac fdk-aac "fdk-aac/aacenc_lib.h" aacEncOpen
 enabled libfontconfig     && require_pkg_config libfontconfig fontconfig "fontconfig/fontconfig.h" FcInit
 enabled libfreetype       && require_pkg_config libfreetype freetype2 "ft2build.h FT_FREETYPE_H" FT_Init_FreeType
-enabled libgsm            && { for gsm_hdr in "gsm.h" "gsm/gsm.h"; do
-                                   check_lib libgsm "${gsm_hdr}" gsm_create -lgsm && break;
-                               done || die "ERROR: libgsm not found"; }
+enabled libgsm            && require libgsm gsm._h gsm_create -lgsm
 enabled libhdcd           && require_pkg_config libhdcd libhdcd "hdcd/hdcd_simple.h" hdcd_new
 enabled libilbc           && require libilbc ilbc.h WebRtcIlbcfix_InitDecode -lilbc
 enabled libkvazaar        && require_pkg_config libkvazaar "kvazaar >= 0.8.1" kvazaar.h kvz_api_get

libavcodec/libgsmdec.c

@@ -27,12 +27,7 @@
 
 // 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"

libavcodec/libgsmenc.c

@@ -27,12 +27,7 @@
 
 // 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/common.h"