xml2-config.in: fix regressions introduced by commit 2f2bf4b2c

One of regressions introduced by commit
2f2bf4b2caa1cb9a4a5039b7a44db101943382d1 aka v2.9.10-rc1~56 is that
cflags and libs variables are used uninitialized, resulting to
the following behaviour:

$ cflags=foo libs=bar sh ./xml2-config.in --prefix
@prefix@
foo bar

Another regression is that the test for these variables is flawed.

Fixes: 2f2bf4b2c ("xml2-config.in: Output CFLAGS and LIBS on the same line")

xml2-config.in

@@ -4,6 +4,8 @@ prefix=@prefix@
 exec_prefix=@exec_prefix@
 includedir=@includedir@
 libdir=@libdir@
+cflags=
+libs=
 
 usage()
 {
@@ -102,7 +104,7 @@ while test $# -gt 0; do
     shift
 done
 
-if test "$cflags" -o "$libs"; then
+if test -n "$cflags$libs"; then
     echo $cflags $libs
 fi