*/*: Fix build when PDF option is enabled

When PDF option is enabled PDF file is generated from Texinfo source
file with texi2pdf, and the command is invoked with '--clear'
option. When the command is invoked with the option, however, it
returns non-zero value even if conversion is (or, at least seems to
be) successful. As a result build of ports fails when the option is
enabled. The command returns zero value if it is invoked without the
option. So fix build by patching Makefile of source tree.

While I'm here, replace CONFLICTS in devel/{forge,forge-devel/Makefile
with CONFLICTS_INSTALL as neither of them interfere with build of the
other.