Make BLASFEO_INCLUDE_DIR user definable

[nielsvd]

Here the BLASFEO_INCLUDE_DIR is forced to be ${BLASFEO_PATH}/include. In blasfeo, the option BLASFEO_HEADERS_INSTALLATION_DIRECTORY is however configurable. It would therefore be more appropriate if BLASFEO_INCLUDE_DIR could be provided as an option. Either by making it an option() explicitly or by writing

set(BLASFEO_INCLUDE_DIR "${BLASFEO_PATH}/include" CACHE STRING "Path to BLASFEO header files.")

instead of

set(BLASFEO_INCLUDE_DIR "${BLASFEO_PATH}/include")

[giaf]

This should be implemented now.