Fix build failures on Ubuntu 13.10.

Automake now insists that we run AM_PROG_AR if we're going to build a
library, and AM_PROG_CC_C_O if we're going to build anything with
extra compile options. Those extra macros seem harmless in previous
versions of automake.

[originally from svn r10053]

unix/configure.ac

@@ -14,6 +14,8 @@ AM_INIT_AUTOMAKE([-Wall -Werror foreign])
 
 AC_PROG_INSTALL
 AC_PROG_RANLIB
+AM_PROG_AR
+AM_PROG_CC_C_O
 
 # Mild abuse of the '--enable' option format to allow manual
 # specification of setuid or setgid setup in pterm.