Add variables to configure qmake and remove relative paths in *.pro.
Set install path for header files with MNE_INCLUDE_DIR and for libs with MNE_LIBRARY_DIR.
Now you can make a header-library-bundle for external applications:
   cd <path to mne-cpp>/MNE
   qmake -recursive MNE_INCLUDE_DIR=<include directory> MNE_LIBRARY_DIR=<library directory>
   make install
Libraries are located in: <library directory> e.g. /opt/lib
Header files are located in: <include directory> e.g. /opt/include
Use of MNE_INCLUDE_DIR to retrieve MNE header files.
Merge pull request #3 from cpieloth/master

Further changes for better library support (UPDATE)
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