autogen.sh:  Use autoreconf instead a custom script
Remove autogenerated INSTALL file
Make maintiner mode enabled by default

See http://blogs.gnome.org/desrt/2011/09/08/am_maintainer_mode-is-not-cool/
configure.ac: Use new modern libtool syntax
configure.ac: Do not use deprecated autoconf macros
configure.ac: store m4 files in m4/ directory
configure.ac: Use AM_SILENT_RULES to enable silent rules

Support silent build rules, requires at least automake-1.11.
Disable by either passing --disable-silent-rules to configure or
passing V=1 to make.
configure.ac: generate xz tarball instead gz one

Its what its used in GNOME servers nowadays
configure.ac: Do not AC_SUBST GTK CFLAGS and LIBS

PKG_CHECK_MODULES already do that for us
configure.ac: Complete AC_INIT information
configure.ac: Compile with -Wall in automake
configure.ac: Check for the math library properly
Use AM_CPPFLAGS instead deprecated INCLUDES
configure: Do not overwrite the user variable CFLAGS

Also we are already setting this in gtkgl/Makefile.am:
AM_CFLAGS = $(GTK_CFLAGS) $(GL_CFLAGS)