rules.mk: output directly to target for LT_COMPILE

For source files that exist in another directory, libtool will by
default put the output file in the current working directory instead of
next to the source file. Our build logic would then `touch` an empty
output file in the place where the actual output should have gone.

Replace the touch logic with an explicit -o option to libtool.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/httpdunit@1796200 13f79535-47bb-0310-9956-ffa450edef68