Use full path for legacyheaders

Advantages:
simpler to setup tooling / IDE (just one path)
show immediately all usage of legacyheaders
No risk of accidentally working includes (together with using relative paths)

Used:
find src/gromacs/ src/programs/ src/testutils/ -name '*.c' -o -name '*.cpp' \
   -o -name '*.h' -o -name '*.cu' -o -name '*.cuh' -o -name '*.pre'|
   xargs -n1 python makeabspath.py

Manual changes to cmake and doxygen files to remove legacyheader include path.

makeabspath.py:
https://gist.github.com/rolandschulz/ee636f517d75e48bc68c

Related to #1557.

Change-Id: I6c1b51a573acfe7b0123485405f2494e97715f53