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.

daemon/Makefile.am

@@ -351,12 +351,14 @@ libvirtd.service: libvirtd.service.in $(top_builddir)/config.status
 
 check-local: check-augeas
 
-test_libvirtd.aug: test_libvirtd.aug.in libvirtd.conf
-	$(AM_V_GEN)$(PERL) $(top_srcdir)/build-aux/augeas-gentest.pl libvirtd.conf $< $@
+AUG_GENTEST = $(PERL) $(top_srcdir)/build-aux/augeas-gentest.pl
+
+test_libvirtd.aug: test_libvirtd.aug.in $(srcdir)/libvirtd.conf
+	$(AM_V_GEN)$(AUG_GENTEST) $(srcdir)/libvirtd.conf $< $@
 
 check-augeas: test_libvirtd.aug
 	$(AM_V_GEN)if test -x '$(AUGPARSE)'; then \
-	  '$(AUGPARSE)' -I $(srcdir) $(srcdir)/test_libvirtd.aug; \
+	  '$(AUGPARSE)' -I $(srcdir) test_libvirtd.aug; \
 	fi