should the word include be part of install path

[hgkamath]

I think the word 'include' in the folder stub should not be path of the make-installed tree

I think the normal convention is to let the user specify the complete install path equivalent to
/usr/include
or
/usr/local/include
or
/usr/include/include/gtk-3.0/gtk (for say some package version gtk3 of some package gtk)

explanation:
start in directory : /full/path/
git clone "https://github.com/michael-lehn/FLENS.git"
mkdir build
mkdir include
cd build
cmake ../FLENS -DCMAKE_INSTALL_PREFIX=cd .. ; pwd/include/flens2015/flens
make install

files install to
/full/path/include/flens2015/flens/include/cxxblas
/full/path/include/flens2015/flens/include/cxxlapack
/full/path/include/flens2015/flens/include/external
/full/path/include/flens2015/flens/include/flens

notice the include stub repeated under flens2015/flens.
flens2015/flens has only a single subfolder 'include'
I believe that its the second 'include', and not the first that was specified on command line of cmake build that is not necessary, please check the convention.

the fix:
change line 5
set(INCLUDE_INSTALL_DIR "${CMAKE_INSTALL_PREFIX}/include" CACHE PATH "Include files install path")
to
set(INCLUDE_INSTALL_DIR "${CMAKE_INSTALL_PREFIX}/flens" CACHE PATH "Include files install path")

the commands...
cmake ../FLENS -DCMAKE_INSTALL_PREFIX=cd .. ; pwd/include/flens2015
make install

... will install as
/full/path/include/flens2015/flens/cxxblas
/full/path/include/flens2015/flens/cxxlapack
/full/path/include/flens2015/flens/external
/full/path/include/flens2015/flens/flens

the gcc include arg needed will be -I /full/path/include/flens2015/flens
A pkg-config conf file can also be made, that will automatically create the compiler include option

the source code will have include < flens/flens.cxx >