maint: don't distribute generated .def files

I ran 'make dist' in the directory left over from ./autobuild.sh
(which was configured for a mingw cross build); the resulting
tarball had more files than 'make dist' on a normal Linux build.
I traced it to the fact that we were distributing a generated
file, but only when configure said the end user had to generate
the file in the first place.  In the process, I noticed that
we had some difference in symbol file names; I added a comment
explaining why the difference exists (after first trying to
normalize the names and hitting VPATH build failures).

* configure.ac (LIBVIRT_QEMU_SYMBOL_FILE): Add some comments.
* src/Makefile.am (EXTRA_DIST): No need to ship a generated file;
particularly since which file is built depends on configure results.

configure.ac

@@ -2676,6 +2676,8 @@ CYGWIN_EXTRA_LIBADD=
 CYGWIN_EXTRA_PYTHON_LIBADD=
 MINGW_EXTRA_LDFLAGS=
 WIN32_EXTRA_CFLAGS=
+dnl libvirt.syms is generated in builddir, but libvirt_qemu.syms is in git;
+dnl hence the asymmetric naming of these two symbol files.
 LIBVIRT_SYMBOL_FILE=libvirt.syms
 LIBVIRT_QEMU_SYMBOL_FILE='$(srcdir)/libvirt_qemu.syms'
 MSCOM_LIBS=

src/Makefile.am

@@ -1564,7 +1564,6 @@ libvirt_qemu_la_LDFLAGS = $(VERSION_SCRIPT_FLAGS)$(LIBVIRT_QEMU_SYMBOL_FILE) \
 			  $(AM_LDFLAGS)
 libvirt_qemu_la_CFLAGS = $(AM_CFLAGS)
 libvirt_qemu_la_LIBADD = libvirt.la $(CYGWIN_EXTRA_LIBADD)
-EXTRA_DIST += $(LIBVIRT_QEMU_SYMBOL_FILE)
 
 lockdriverdir = $(libdir)/libvirt/lock-driver
 lockdriver_LTLIBRARIES =