[build] Fix --enable-pie so it can actually be disabled

Signed-off-by: Paul Jakma <paul@quagga.net>
diff --git a/configure.ac b/configure.ac
index 547b686..6d824f5 100755
--- a/configure.ac
+++ b/configure.ac
@@ -1390,10 +1390,8 @@
 AC_SUBST(CONFDATE)
 
 dnl Conditionally enable PIE support for GNU toolchains.
-enable_pie=yes
-
 AC_ARG_ENABLE(pie, AS_HELP_STRING([--disable-pie], [Do not build tools as a Position Independent Executables]))
-if test "$enable_pie" = "yes"; then
+if test "$enable_pie" != "no"; then
   AC_CACHE_CHECK([whether $CC accepts PIE flags], [ap_cv_cc_pie], [
     save_CFLAGS=$CFLAGS
     save_LDFLAGS=$LDFLAGS