meson: better error for wcap dep cairo

Add human-friendly error message.

Cairo is a hard dependency on the whole at least because tests seem to
require it, but this will help if someone adds an option to disable
building tests to get rid of Cairo.

Signed-off-by: Pekka Paalanen <pq@iki.fi>

wcap/meson.build

@@ -7,15 +7,15 @@ srcs_wcap = [
 	'wcap-decode.c',
 ]
 
-deps_wcap = [
-	dep_libm,
-	dependency('cairo'),
-]
+wcap_dep_cairo = dependency('cairo', required: false)
+if not wcap_dep_cairo.found()
+	error('wcap requires cairo which was not found. Or, you can use \'-Dwcap-decode=false\'.')
+endif
 
 executable(
 	'wcap-decode',
 	srcs_wcap,
 	include_directories: include_directories('..'),
-	dependencies: deps_wcap,
+	dependencies: [ dep_libm, wcap_dep_cairo ],
 	install: true
 )