Use pkg-config to determine where to find the devmapper library

Use pkg-config to determine where to find the devmapper library

Fedora and Red Hat puts the devmapper library in different locations
compared to Debian, so we use pkg-config.  Unfortunately Debian's
devmapper.pc file is buggy (See Debian Bug #390243), so we have to
work around it.

Historically, e2fsprogs has tried not to depend on pkg-config, since
its answers are so often **wrong** (the Debian bug has been ignored
for over a year), so I'm hoping I'm not going to regret this.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>