Remove AC_USE_SYSTEM_EXTENSIONS, seems it confuses recent automake/autoconf...

Remove AC_USE_SYSTEM_EXTENSIONS, seems it confuses recent automake/autoconf which sets a wrong path to install.sh

No regression with automake 1.11 / autoconf 2.63.

configure.ac.in

@@ -12,7 +12,6 @@ m4_define([xfce4_screenshooter_version_tag], [])
 m4_define([xfce4_screenshooter_version], [xfce4_screenshooter_version_major().xfce4_screenshooter_version_minor().xfce4_screenshooter_version_micro()ifelse(xfce4_screenshooter_version_nano(), [], [], [.xfce4_screenshooter_version_nano()])ifelse(xfce4_screenshooter_version_tag(), [git], [xfce4_screenshooter_version_tag()-xfce4_screenshooter_version_build()], [xfce4_screenshooter_version_tag()])])
 
 AC_INIT([xfce4-screenshooter], [xfce4_screenshooter_version], [http://bugzilla.xfce.org/], [xfce4-screenshooter])
-AC_USE_SYSTEM_EXTENSIONS()
 
 dnl ***************************
 dnl *** Initialize automake ***