meson: Fix missing GSM_HEADER_IN_SUBDIR logic

Until now, this hadn't been translated from autoconf yet.

ext/gsm/meson.build

@@ -2,19 +2,23 @@ if get_option('gsm').disabled()
   subdir_done()
 endif
 
+gsm_cargs = []
+
 gsm_dep = cc.find_library('gsm', required : get_option('gsm'))
 have_gsm_create = cc.has_header_symbol('gsm.h', 'gsm_create')
 if not have_gsm_create
   have_gsm_create = cc.has_header_symbol('gsm/gsm.h', 'gsm_create')
   if not have_gsm_create and get_option('gsm').enabled()
     error('GSM plugin is enabled: found libgsm but no headers')
+  else
+    gsm_cargs += ['-DGSM_HEADER_IN_SUBDIR']
   endif
 endif
 
 if gsm_dep.found() and have_gsm_create
   gstgsm = library('gstgsm',
     ['gstgsm.c', 'gstgsmenc.c', 'gstgsmdec.c'],
-    c_args : gst_plugins_bad_args,
+    c_args : gst_plugins_bad_args + gsm_cargs,
     include_directories : [configinc],
     dependencies : [gstaudio_dep, gsm_dep],
     install : true,