That allows a build system to customize compiler and linker and pass
extra flags to these.
Remove `-g` as default flag and enable it only when `DEBUG` variable is
set.
qubes-rpc has its own Makefile that's responsible for building some
executables. The root Makefile was installing qubes-rpc files. To make
qubes-rpc a bit more indepdent from core-agent root Makefile and to ease
potential maintainer work on packaging qubes-rpc separately, the
installation has been moved to qubes-rpc Makefile. Moreover that should
make the Makefiles easier to read and maintain.
This makes the qfile packing code reusable, for example for some dom0
tool. Now qfile-agent.c is only an interface for underlying library.
QubesOS/qubes-issues#1324
