Add a PREFIX detector to ComplxApp.cpp

Needs testing; upon success, the function needs to be moved elsewhere
and its prototype copied into a header file, so that ComplxFrame can use it.
Revert "Add a PREFIX detector to ComplxApp.cpp"

This reverts commit 2ad5849768ab25460811253632a65e3f053ce6d6.
Add a PREFIX cpp symbol to the complx build tree, and use it.

Untested.
Actually use expanded concatenation for PREFIX.

Tested. Should work now.
Give the new macros better names.
Add one invocation of _() to ComplxApp.cpp.
Merge pull request #15 from mmirate/master

Properly handle PREFIX changes