gp: change from xalan to xsltproc

We have been using an outdated version of xalan, which is what is
installed using apt in Ubuntu. If you run Fedora, you will receive the
latest version. The executable name has changed from "xalan" to "Xalan"
and the supported command line arguments are also different. Therefore
supporting different versions of xalan across distros can be
challenging.

One alternative is xsltproc. It looks like it works on both Ubuntu and
Fedora, so let's swap out xalan for xsltproc.

Signed-off-by: Joakim Bech <[email protected]>
Acked-by: Etienne Carriere <[email protected]>
Acked-by: Jens Wiklander <[email protected]>
Acked-by: Jerome Forissier <[email protected]>
Tested-by: Jerome Forissier <[email protected]> (Ubuntu 20.04)
Tested-by: Joakim Bech <[email protected]> (Fedora 41)
Tested-by: Joakim Bech <[email protected]> (Ubuntu 22.04)

host/xtest/CMakeLists.txt

@@ -74,8 +74,8 @@ if (WITH_GP_TESTS)
 	macro(__GEN_GP_FILE outfile xmldir name basedir)
 		add_custom_command(
 			OUTPUT	${outfile}
-			COMMAND	xalan -in ${basedir}/${name}.xml
-				      -xsl gp/${name}.xsl -out ${outfile}
+			COMMAND	xsltproc -o ${outfile} gp/${name}.xsl
+							${basedir}/${name}.xml
 			DEPENDS ${basedir}/${name}.xml gp/${name}.xsl
 		)
 	endmacro(__GEN_GP_FILE)