btrfs-progs: build: configure.ac hard-codes the pkg-config command

btrfs-progs: build: configure.ac hard-codes the pkg-config command

Right now the pkg-config command is hard-coded in configure.ac, which may
result in build errors on system and cross environments that have prefixed
toolchains, e.g. /usr/x86_64-pc-linux-gnu/bin/x86_64-pc-linux-gnu-pkg-config.

Please see the attached patch, it has been written a while ago but it seems it
hasn't been submitted for upstream inclusion.
0001-configure.ac-Consistently-use-PKG_CONFIG.txt

Submitted by Timo Gurr.

Author: Wulf C. Krueger <philantrop@exherbo.org>
Issue: #101
Signed-off-by: David Sterba <dsterba@suse.com>