build: export full cflags in pkgconfig file
build: export full cflags in pkgconfig file

The cflags needed to build seastar will also be needed to build the
application, and duplicating them in the application is annoying
(especially the warning detection logic that changes frequently).

Expose all compiler flags via pkgconfig; this requires making -I flags
use absolute paths so they are cwd independent.
Message-Id: <35add9c2ff6de56de85afe3292fc6b32a94ab803@scylladb.com>

syuu1228/seastar,dreamsxin/seastar,cloudius-systems/seastar,avikivity/seastar,avikivity/seastar,cloudius-systems/seastar,avikivity/seastar,syuu1228/seastar,scylladb/seastar,dreamsxin/seastar,syuu1228/seastar,dreamsxin/seastar,scylladb/seastar,cloudius-systems/seastar,scylladb/seastar