Ensure backslashes in props file aren't mangled.

Author: jnthn

tools/build/gen-jvm-properties.pl

@@ -22,6 +22,8 @@
 
 $thirdPartyJars .= "${cpsep}${jardir}/nqp-runtime.jar${cpsep}${libdir}/nqp.jar";
 
+s/\\/\\\\/g for ($prefix, $thirdPartyJars, $libdir);
+
 print <<"END";
 # This file automatically generated by $0