libgm2: Adjust 'autogen.sh' to 'ACLOCAL_AMFLAGS', and simplify

Specifying explicit '-I ..' before '-I ../config' is what (most) other GCC
components do.  Specifying '-I .' is not necessary.

With the order of '-I's aligned, 'autogen.sh' and plain 'autoreconf' then
produce identical results.

	libgm2/
	* autogen.sh: For 'aclocal', 'autoreconf', remove '-I .',
	add '-I ..'.
	* Makefile.am (ACLOCAL_AMFLAGS): Remove '-I .'.
	* libm2cor/Makefile.am (ACLOCAL_AMFLAGS): Likewise.
	* libm2iso/Makefile.am (ACLOCAL_AMFLAGS): Likewise.
	* libm2log/Makefile.am (ACLOCAL_AMFLAGS): Likewise.
	* libm2min/Makefile.am (ACLOCAL_AMFLAGS): Likewise.
	* libm2pim/Makefile.am (ACLOCAL_AMFLAGS): Likewise.
	* aclocal.m4: Regenerate.
	* Makefile.in: Likewise.
	* libm2cor/Makefile.in: Likewise.
	* libm2iso/Makefile.in: Likewise.
	* libm2log/Makefile.in: Likewise.
	* libm2min/Makefile.in: Likewise.
	* libm2pim/Makefile.in: Likewise.