build changes: --with-elfutils option

[Roland McGrath <roland at redhat dot com>]

I've committed changes relating to PR 1916.  This is to make it easier to
build elfutils locally in the systemtap build, as we have done in the RPM
builds.  

configure now accepts an option --with-elfutils=SRCDIR, where SRCDIR is a
directory containing the elfutils source tree, either an absolute directory
name or one relative to the systemtap source directory where configure is.
You'll see it also run elfutils' configure script at the end.

After you have configured this way, "make" in your systemtap build
directory will do a local elfutils build first.  If you change elfutils
source files, you'll need to remove the file stamp-elfutils in your
systemtap build directory to be sure the next "make" rebuilds everything.

Note that these locally-built libraries will not be installed on your
system.  stap will be linked to expect them in ${pkglibdir},
i.e. /usr/lib/systemtap if you used --prefix=/usr.  They won't be there
before you do "make install", so you'll need to set LD_LIBRARY_PATH to run
your local stap binary.  The runtest.sh script takes care of this for
running the test suite for "make check".  You can use something like:

	export LD_LIBRARY_PATH=$PWD/lib-elfutils:$PWD/lib-elfutils/systemtap

The systemtap.spec.in file now uses this configure option, so you can see
an example use there.  Someone should update the README instructions.


Thanks,
Roland