Allow to build with tests and benchmarks

[franziskuskiefer]

This option collides with --tests.

$ ./mach build --tests --benchmarks

...

CMake Error at CMakeLists.txt:436 (add_library):
  add_library cannot create target "hacl_cpu_features" because another target
  with the same name already exists.  The existing target is created in
  source directory "/home/duesee/repos/cryspen/hacl-packages".  See
  documentation for policy CMP0002 for more details.


-- Configuring incomplete, errors occurred!

Originally posted by @duesee in #74 (comment)

[franziskuskiefer]

depends on #116