[[PATCH v2 for-4.14]] m4: use test instead of []

[Wei Liu <wl@xxxxxxx>]

It is reported that [] was removed by autoconf, which caused the
following error:

  ./configure: line 4681: -z: command not found

Switch to test. That's what is used throughout our configure scripts.
Also put the variable expansion in quotes.

Signed-off-by: Wei Liu <wl@xxxxxxx>
Reported-by: Bertrand Marquis <Bertrand.Marquis@xxxxxxx>
Fixes: 8a6b1665d987 ("configure: also add EXTRA_PREFIX to {CPP/LD}FLAGS")
Signed-off-by: Wei Liu <wl@xxxxxxx>
---
v2: Address Ian's comment.

Run autogen.sh before committing.

Cc: Bertrand Marquis <Bertrand.Marquis@xxxxxxx>
Cc: Roger Pau Monne <roger.pau@xxxxxxxxxx>
Cc: paul@xxxxxxx
---
 m4/set_cflags_ldflags.m4 | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/m4/set_cflags_ldflags.m4 b/m4/set_cflags_ldflags.m4
index 08f5c983cc63..23706e256b42 100644
--- a/m4/set_cflags_ldflags.m4
+++ b/m4/set_cflags_ldflags.m4
@@ -15,7 +15,7 @@ for ldflag in $APPEND_LIB
 do
     APPEND_LDFLAGS="$APPEND_LDFLAGS -L$ldflag"
 done
-if [ ! -z $EXTRA_PREFIX ]; then
+if test ! -z "$EXTRA_PREFIX" ; then
     CPPFLAGS="$CPPFLAGS -I$EXTRA_PREFIX/include"
     LDFLAGS="$LDFLAGS -L$EXTRA_PREFIX/lib"
 fi
-- 
2.20.1