Replace obsolete automake AM_CONFIG_HEADER.

This fixes the following complaint by autoconf 2.69-1, automake 1.13.1-1.

: configure.ac:80: error: 'AM_CONFIG_HEADER': this macro is obsolete.
: You should use the 'AC_CONFIG_HEADERS' macro instead.
: /usr/share/aclocal-1.13/obsolete-err.m4:12: AM_CONFIG_HEADER is expan
: configure.ac:80: the top level

Automake 1:1.11.3-1ubuntu2, autoconf 2.68-1ubuntu2 don't even emit a warning
without, and work just fine with this patch.

Signed-off-by: Christian Vogel <vogelchr@vogel.cx>
Signed-off-by: Steve Markgraf <steve@steve-m.de>
Merge branch 'master' of git://git.osmocom.org/rtl-sdr

Conflicts:
	configure.ac