Automake 1.7.2 is the oldest version working with xrdp

Automake 1.6.x cannot deal with conditional EXTRA_LIBS. Automake 1.7 and
1.7.1 cannot deal with line continuation in SUBDIRS.

It is important to have correct requirements to know what Automake
features can be used.

configure.ac

@@ -3,7 +3,7 @@
 AC_PREREQ(2.59)
 AC_INIT([xrdp], [0.9.0], [xrdp-devel@googlegroups.com])
 AC_CONFIG_HEADERS(config_ac.h:config_ac-h.in)
-AM_INIT_AUTOMAKE([1.6 foreign])
+AM_INIT_AUTOMAKE([1.7.2 foreign])
 AC_CONFIG_MACRO_DIR([m4])
 AC_PROG_CC
 AC_C_CONST