Remove an obsolete macro in configure.in

Remove an obsolete macro in configure.in

Replaced AM_CONFIG_HEADER with AC_CONFIG_HEADERS

lano1106@whippet2 ~/dev/alsa-lib $ ./gitcompile
aclocal: warning: autoconf input should be named 'configure.ac', not 'configure.in'
configure.in:56: error: 'AM_CONFIG_HEADER': this macro is obsolete.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 expanded from...
configure.in:56: the top level
autom4te: /usr/bin/m4 failed with exit status: 1
aclocal: error: echo failed with exit status: 1

Signed-off-by: Olivier Langlois <olivier@trillion01.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>