Revert "Remove build time variables"

This reverts commit 05e95a8f.

gitlab/conda_build_config.yaml

@@ -53,6 +53,17 @@ bob_devel:
 beat_devel:
   - 2018.10.16
 
+# The build time only dependencies (build requirements).
+# Updating these to the latest version all the time is OK and a good idea.
+# These versions should match the versions inside bob-devel as well (if they
+# overlap) so update them in both places.
+cmake:
+  - 3.12.2
+pkg_config:
+  - 0.29.2
+cython:
+  - 0.28.1
+
 # The dependencies that we link against (host requirements).
 # Ideally we want to build against the oldest possible version of packages when
 # we are linking against them. It is best to keep this in sync with: