Revert "Makefile: make mkdir -p step for source dir an explicit step …

[rawlingsj]

…so it can be overridden"

We're getting failed release builds, think it's due to this change. Let's roll back to get green again and reimplement this.

This reverts commit 6e8612a.

[rawlingsj]

/cc @kaniini

[dlorenc]

My theory for the problem is that the change here:

is making it so that the directory ${sourcedir} is getting created for every package, and because it's listed as a dependency for for $(pkgtarget), it's busting the cache and triggering every package to get rebuilt (unless it already has a directory). This can be observed in the failing logs, where we attempt to build 61 packages before failing on cmake: https://pipelines.actions.githubusercontent.com/serviceHosts/969e5abc-ffab-48b3-bd54-b87797129033/_apis/pipelines/1/runs/15821/signedlogcontent/2?urlExpires=2023-04-18T11%3A00%3A03.3656729Z&urlSigningMethod=HMACV1&urlSignature=NPcXAbGBpnFvOfSLTI1lxAySHjfug2RnBsjU0%2F38BtM%3D

These builds are ordered by their order in packages.txt, but with packages that already have directories missing:

mpfr is first, when gmp is before it. Then mpc, isl, zlib, flex, but no glibc.