meson-build: fix the libclang check

Use args argument in cc.has_header because it doesn't support prefix.

Add dirs argument to cc.find_library because we have to add extra
search paths to find libclang on systems not installing it in the
default search path. Replace 'llvm-config --ldflags' with
'llvm-config --libdir' because cc.find_library uses dirs argument
instead of args.

Remove link_args from clang_dep because using cc.find_library makes
meson use the full path to the library. -L flags are not needed here.

https://bugzilla.gnome.org/show_bug.cgi?id=782296