Add support for autoconf macros in m4/

Needed by commit 972a34ec2c79d241318af24389b8ee042d10556a.

Use autoreconf in autogen.sh instead of the handwritten version,
it's simpler, and also updates things that we weren't handling.

Drop the hand-written INSTALL file.  Its information content was
~zero, and autotools wants to overwrite it with its own version,
so don't fight that, just .gitignore it.
diff --git a/.gitignore b/.gitignore
index 27a956b..794b4a7 100644
--- a/.gitignore
+++ b/.gitignore
@@ -15,6 +15,7 @@
 config.sub
 configure
 depcomp
+INSTALL
 install-sh
 .deps
 .libs
@@ -25,6 +26,11 @@
 libtool
 ltmain.sh
 missing
+m4/libtool.m4
+m4/ltoptions.m4
+m4/ltsugar.m4
+m4/ltversion.m4
+m4/lt~obsolete.m4
 opus_compare
 opus_demo
 repacketizer_demo