wmlib: Remove install-exec-local target.

wmlib: Remove install-exec-local target.

This was made redundant by defining pkgconfigdir and pkgconfig_DATA.  On
some systems (with automake < 1.15), this caused a build failure, e.g., [1]:

/usr/bin/install -c -m 644 wmlib.pc /«BUILDDIR»/wmaker-0.95.7+201601230517/
 debian/tmp/usr/lib/x86_64-linux-gnu/pkgconfig/
/usr/bin/install -c -m 644 wmlib.pc '/«BUILDDIR»/wmaker-0.95.7+201601230517/
 debian/tmp/usr/lib/x86_64-linux-gnu/pkgconfig'
/usr/bin/install: cannot create regular file '/«BUILDDIR»/
 wmaker-0.95.7+201601230517/debian/tmp/usr/lib/x86_64-linux-gnu/pkgconfig/
 wmlib.pc': File exists

Also change the definition of DISTCLEANFILES to pkgconfig_DATA to match
the Makefiles for WINGs and wrlib.

[1] https://launchpadlibrarian.net/234989800/
 buildlog_ubuntu-precise-amd64.wmaker_0.95.7+201601230517-0ppa201601211606~
 ubuntu12.04.1_BUILDING.txt.gz