fix warning when SOCL_OCL_LIB_OPENCL is empty

configure.ac

@@ -2325,7 +2325,7 @@ AM_CONDITIONAL([STARPU_USE_SOCL], [test "x$build_socl" = "xyes"])
 
 if test "$build_socl" = "yes" ; then
    AC_CHECK_FUNCS([clGetExtensionFunctionAddressForPlatform])
-   if test -n "$SOCL_OCL_LIB_OPENCL" -a -f $SOCL_OCL_LIB_OPENCL ; then
+   if test -n "$SOCL_OCL_LIB_OPENCL" -a -f "$SOCL_OCL_LIB_OPENCL" ; then
       run_socl_check=yes
       SOCL_OCL_LIB_OPENCL_DIR=$(dirname $SOCL_OCL_LIB_OPENCL)
       AC_SUBST(SOCL_OCL_LIB_OPENCL_DIR)