configure.ac: do not use deprecated macros - AC_HELP_STRING AC_ERROR

Replace with AS_HELP_STRING and AC_MSG_ERROR respectively, as spotted by
autoupdate.

Note that the suggested AC_CANONICAL_SYSTEM > AC_CANONICAL_TARGET change
is not addressed here since that requires very extensive testing.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
diff --git a/configure.ac b/configure.ac
index b4ea40f..39da5b4 100644
--- a/configure.ac
+++ b/configure.ac
@@ -442,7 +442,7 @@
 CFLAGS=$save_CFLAGS
 
 AC_ARG_ENABLE(pwr8,
-   [AC_HELP_STRING([--disable-pwr8-inst],
+   [AS_HELP_STRING([--disable-pwr8-inst],
                    [disable POWER8-specific instructions])],
    [enable_pwr8=$enableval], [enable_pwr8=auto])
 
@@ -2379,7 +2379,7 @@
 dnl We need to validate some needed dependencies for renderonly drivers.
 
 if test "x$HAVE_GALLIUM_ETNAVIV" != xyes -a "x$HAVE_GALLIUM_IMX" = xyes  ; then
-    AC_ERROR([Building with imx requires etnaviv])
+    AC_MSG_ERROR([Building with imx requires etnaviv])
 fi
 
 dnl