cmake/general: make BUILD_TESTS option explicit
cmake/general: make BUILD_TESTS option explicit

This allows to get rid of stupid code like this

  if(DEFINED BUILD_TESTS AND NOT ${BUILD_TESTS})
    ...
  endif()

amd just use:

  if(NOT BUILD_TESTS)
    ...
  endif()

Change-Id: Ib2f6d2c31fbeb304087b320e8c860a460de86c14
Reviewed-on: http://gerrit.aldebaran.lan:8080/1959
Reviewed-by: llec <42f1a1c02cacc0f5b0188f0127ea77bedfdcf21a@aldebaran-robotics.com>
Tested-by: llec <42f1a1c02cacc0f5b0188f0127ea77bedfdcf21a@aldebaran-robotics.com>
Reviewed-by: dmerejkowsky <1e4b13000fa8777df4cefd2fd8215187e3deb062@aldebaran-robotics.com>
Tested-by: dmerejkowsky <1e4b13000fa8777df4cefd2fd8215187e3deb062@aldebaran-robotics.com>

dmerejkowsky/qibuild,dmerejkowsky/qibuild,dmerejkowsky/qibuild,aldebaran/qibuild,aldebaran/qibuild,dmerejkowsky/qibuild,aldebaran/qibuild,dmerejkowsky/qibuild,aldebaran/qibuild