configure.ac: use a separate folder for Autoconf-generated files

Autoconf generates several files in root folder of the project.
Keep the root folder cleaner by specifying subfolder 'build-aux'.
Align .gitignore accordingly.

Signed-off-by: R. Diez <rdiezmail-openocd@yahoo.de>
Change-Id: Ied87faba495d9eeb8f98e78c2e2b7e7e596febfb
Reviewed-on: http://openocd.zylin.com/6236
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

.gitignore

@@ -53,27 +53,23 @@ doc/openocd.toc
 doc/openocd.tp
 doc/openocd.vr
 doc/version.texi
-texinfo.tex
 src/openocd
 src/openocd.exe
 
 # configure/autotools output
+/build-aux/
 aclocal.m4
 autom4te.cache
-compile
-config.*
+config.h
+config.log
+config.status
 configure
-depcomp
 doxygen
 doxygen.log
 Doxyfile
-install-sh
 libtool
-ltmain.sh
 Makefile
 !contrib/loaders/**/Makefile
-mdate-sh
-missing
 stamp-h1
 stamp-vti
 INSTALL