meson: create a srtp dependency

The dependency used in srtp_lib is not carried over in the module.
But it may contain compile_args that are not accessible another way.
We put everything need to use the srtp library in a meson dependency
that can be used from many places.

modules/access/rtp/meson.build

@@ -9,10 +9,13 @@ if gcrypt_dep.found()
                     files('srtp.c', 'srtp.h'),
                     dependencies: [gcrypt_dep],
                     include_directories: [vlc_include_dirs])
-    srtp_rtp_c_args = ['-DHAVE_SRTP']
+    srtp_dep = declare_dependency(
+        link_with: [srtp_lib],
+        include_directories: [include_directories('.')],
+        dependencies: [gcrypt_dep],
+        compile_args: ['-DHAVE_SRTP'])
 else
-    srtp_lib = []
-    srtp_rtp_c_args = []
+    srtp_dep = disabler()
 endif
 
 vlc_modules += {
@@ -27,9 +30,8 @@ vlc_modules += {
             'rtp.c',
             'rtp.h',
         ),
-    'dependencies': [socket_libs],
-    'link_with': [rtp_lib, srtp_lib],
-    'c_args': srtp_rtp_c_args,
+    'dependencies': [socket_libs, srtp_dep],
+    'link_with': [rtp_lib],
 }
 
 vlc_modules += {