build: resolve cmake processing error on Linux

[jengelh]

Introduced by commit 959f50eb79af5b1e08b3387d33b5bf91c7c8b530 was this odd build failure:

$ cmake . && make dsda-doom-wad
-- Configuring done (1.1s)
-- Generating done (0.0s)
-- Build files have been written to: ~/dsda-doom/prboom2 [ 75%] Built target rdatawad
[100%] Built target dsda-doom-wad
$ make
[  3%] Built target rdatawad
[  4%] Built target dsda-doom-wad
src/CMakeFiles/dsda-doom.dir/build.make:3950: warning: overriding recipe for target 'dsda-doom' src/CMakeFiles/dsda-doom.dir/build.make:3940: warning: ignoring old recipe for target 'dsda-doom'

It appears that the path suffix at some points gets a match with /usr as an anchor. Specifying the full build path seems to sidestep the issue.

As the whole PATH_SUFFIX thing seems to be a Windows-centric addition (under Linux it finds the system libmad anyway), please give this a test on that platform.

Could fix #434.