Create the common location for all m4 autotool build scripts, build-aux/m4.

Update .gitignore.

.gitignore

@@ -8,25 +8,32 @@ src/bitcoin-tx
 src/test/test_bitcoin
 src/qt/test/test_bitcoin-qt
 
+# autoreconf
 Makefile.in
 aclocal.m4
 autom4te.cache/
+build-aux/config.guess
+build-aux/config.sub
+build-aux/depcomp
+build-aux/install-sh
+build-aux/ltmain.sh
+build-aux/m4/libtool.m4
+build-aux/m4/lt~obsolete.m4
+build-aux/m4/ltoptions.m4
+build-aux/m4/ltsugar.m4
+build-aux/m4/ltversion.m4
+build-aux/missing
+build-aux/compile
+build-aux/test-driver
 config.log
 config.status
 configure
+libtool
 src/config/bitcoin-config.h
 src/config/bitcoin-config.h.in
 src/config/stamp-h1
-src/build-aux/
 share/setup.nsi
 share/qt/Info.plist
-# Libtool
-libtool
-src/m4/libtool.m4
-src/m4/ltoptions.m4
-src/m4/ltsugar.m4
-src/m4/ltversion.m4
-src/m4/lt~obsolete.m4
 
 src/univalue/gen