build: fix testing of augeas files in VPATH builds

Without this fix, a VPATH build (such as used by ./autobuild.sh)
fails with messages like:

make[3]: Entering directory `/home/remote/eblake/libvirt-tmp2/build/daemon'
../../build-aux/augeas-gentest.pl libvirtd.conf ../../daemon/test_libvirtd.aug.in test_libvirtd.aug
cannot read libvirtd.conf: No such file or directory at ../../build-aux/augeas-gentest.pl line 38.

Since the test files are not part of the tarball, we can generate
them into the build dir, but rather than create a subdirectory
just for the test file, it is easier to test them directly in
libvirt.git/src.

* daemon/Makefile.am (AUG_GENTEST): Factor out definition.
(test_libvirtd.aug): Look for correct file.
* src/Makefile.am (AUG_GENTEST): Use $(PERL).
(qemu/test_libvirtd_qemu.aug, lxc/test_libvirtd_lxc.aug)
(locking/test_libvirt_sanlock.aug): Rename to avoid subdirectories.
(check-augeas-qemu, check-augeas-lxc, check-augeas-sanlock): Reflect
location of built tests.
* configure.ac (PERL): Substitute perl.

configure.ac

@@ -2243,6 +2243,9 @@ AM_CONDITIONAL([WITH_PYTHON], [test "$with_python" = "yes"])
 AC_SUBST([PYTHON_VERSION])
 AC_SUBST([PYTHON_INCLUDES])
 
+dnl Allow perl overrides
+AC_PATH_PROG([PERL], [perl])
+
 AC_ARG_ENABLE([with-test-suite],
               AC_HELP_STRING([--with-test-suite], [build test suite by default @<:@default=check@:>@]),
               [case "${withval}" in