Blasfeo as reusable CMake package

[roversch]

I've been reading up on CMake packages, and as an exercise I rewrote the blasfeo CMake, based on the most current Makefile. You can use it as follows:

cmake_minimum_required(VERSION 3.0)
project(test_blasfeo)

list(APPEND CMAKE_PREFIX_PATH "path/to/blasfeo/installation")
find_package(blasfeo)

add_executable(example calculation.c)
target_link_libraries(example blasfeo)

For ease of use, I also added the possibility for it to work with the CMake GUI and ccmake, with drop-down menus for the different choices of target, BLAS implementation etc.

UPDATE:

• Minimize GCC-specific flags (down to 2, -m64 for MKL and -std=c99 for BLIS)
• Add all sources in one big file, as it was before
• Adding compile_definitions instead of compiler flags leads to robuster builds

[roversch]

@giaf I updated the PR (see above), please review and merge, or add comments.

[roversch]

@giaf since we're doing pull requests now, any comments on this one?

By the way, I wanted to fix the conflicts, but I can't, since you revoked my access since starting this PR. If you want, we can go over them together.