Optionally use an external popt library

The one imported into pkg-config has seen very little maintenance, so
let's give users the option to use the upstream version. The default is
to use the included sources, but it can search for the system library
using --with-installed-popt.

Makefile.am

@@ -6,9 +6,16 @@ GLIB_SUBDIR = glib-1.2.10
 included_glib_includes = -I./glib-1.2.10
 pkg_config_LDADD=glib-1.2.10/libglib.la
 endif
+
+if USE_INSTALLED_POPT
+pkg_config_LDADD += $(POPT_LIBS)
+else
 pkg_config_LDADD += popt/libpopt.la
 popt_includes = -I./popt
-SUBDIRS = $(GLIB_SUBDIR) popt check
+POPT_SUBDIR = popt
+endif
+
+SUBDIRS = $(GLIB_SUBDIR) $(POPT_SUBDIR) check
 
 m4dir = $(datadir)/aclocal
 m4_DATA = pkg.m4

configure.in

@@ -135,6 +135,22 @@ else
   AC_CONFIG_SUBDIRS(glib-1.2.10)
 fi # !native_win32
 
+AC_ARG_WITH([installed-popt],
+  [AS_HELP_STRING([--with-installed-popt], [use installed popt library])],
+  [with_installed_popt="$withval"],
+  [with_installed_popt=no])
+if test "x$with_installed_popt" = xyes; then
+  _save_libs="$LIBS"
+  AC_CHECK_LIB([popt], [poptGetContext], [:],
+    [AC_MSG_ERROR([could not link to installed popt library])])
+  LIBS="$_save_libs"
+  POPT_LIBS=-lpopt
+else
+  POPT_LIBS=""
+fi
+AC_SUBST([POPT_LIBS])
+AM_CONDITIONAL([USE_INSTALLED_POPT], [test "x$with_installed_popt" = xyes])
+
 AC_FUNC_ALLOCA
 
 AC_CHECK_FUNCS(setresuid setreuid,break)