Fix syntax errors

1) It is not safe to use the [ command in configure.in; m4 will eat
the [ and ] and they will expand to nothing. (Just look in the
configure script produced prior to this commit.) Use the test command
instead, as elsewhere in the file.

2) Quote the variable being tested for emptiness with test
-z. Otherwise, if it *is* empty, the -z will apply to (and eat) the
following word, "then", which is not what is intended, and which also
causes a syntax error.

configure.in

@@ -2009,7 +2009,7 @@ if test "$_os" = "Darwin" ; then
         MAC_OS_X_VERSION_MIN_REQUIRED="1050"
         AC_MSG_WARN([Building with a minimum version requirement > 10.4 break 10.4 compatibility. do not use for deliverable build])
         echo "Building with a minimum version requirement > 10.4 break 10.4 compatibility. do not use for deliverable build" >>warn
-        if [ -z $save_CC ] ; then
+        if test -z "$save_CC" ; then
             CC="gcc-4.2 -m32"
             CXX="g++-4.2 -m32"
         fi
@@ -2018,7 +2018,7 @@ if test "$_os" = "Darwin" ; then
         MAC_OS_X_VERSION_MIN_REQUIRED="1060"
         AC_MSG_WARN([Building with a minimum version requirement > 10.4 break 10.4 compatibility. do not use for deliverable build])
         echo "Building with a minimum version requirement > 10.4 break 10.4 compatibility. do not use for deliverable build" >>warn
-        if [ -z "$save_CC" ] ; then
+        if test -z "$save_CC" ; then
             CC="gcc-4.2 -m32"
             CXX="g++-4.2 -m32"
         fi
@@ -2027,7 +2027,7 @@ if test "$_os" = "Darwin" ; then
         MAC_OS_X_VERSION_MIN_REQUIRED="1070"
         AC_MSG_WARN([Building with a minimum version requirement > 10.4 break 10.4 compatibility. do not use for deliverable build])
         echo "Building with a minimum version requirement > 10.4 break 10.4 compatibility. do not use for deliverable build" >>warn
-        if [ -z "$save_CC" ] ; then
+        if test -z "$save_CC" ; then
             CC="gcc-4.2 -m32"
             CXX="g++-4.2 -m32"
         fi