Fix test suite detection.

https://github.com/proftpd/proftpd/pull/839

Reported-by: Toralf Förster
Bug: https://bugs.gentoo.org/697782
--- a/configure.in
+++ b/configure.in
@@ -991,7 +991,7 @@ AC_ARG_ENABLE(tests,
     [enable unit tests (default=no)])
   ],
   [
-    if test "$enableval" != xno ; then
+    if test x"$enableval" != xno ; then
       AC_CHECK_HEADERS(check.h)
 
       AC_CHECK_LIB(check, tcase_create,
--- a/configure
+++ b/configure
@@ -20639,7 +20639,7 @@ ENABLE_TESTS="\"\""
 # Check whether --enable-tests was given.
 if test "${enable_tests+set}" = set; then
   enableval=$enable_tests;
-    if test "$enableval" != xno ; then
+    if test x"$enableval" != xno ; then
 
 for ac_header in check.h
 do