fix installation of main module headers

see: http://code.opencv.org/issues/2897

The order of the 2 conditions is really important, as every MATCHES
operator sets ${CMAKE_MATCH_1}, which is needed inside the condition
block. Ideally, this should be done using a single regex with a
negative forward lookahead, but CMake doesn't seem to support this
Merge pull request #797 from vhdirk:installheaders