build: datetime insource build

Previous version of tzcode/CMakeLists.txt leaves untracked files under
source directory src/lib/tzcode/tz/. We have changed slightly an approach
used to checkout and build IANA github sources under build/ directory,
not in ${PROJECT_BINARY_DIR} (which may be pointing not to out-of-source
build directory, but rather to the repository root itself, if insource
build used).

Closes #7173

NO_CHANGELOG=infra
NO_DOC=infra
NO_TEST=infra

(cherry picked from commit 5f143288)