Fix up top-level build changes

Description

This MR is a fix-up to the top-level build changes in !1902 (merged):

1. If a LAL library is not being built, do not enforce that its environment variables for CFLAGS and data/Octave/Python paths are set; otherwise partial top-level build are not possible (#554 (closed)).
2. Also need to propagate CFLAGS through dependency chain so that libraries can find the include/lal directory for their dependent libraries.

To make sure that partial top-level builds are not broken in future, I've added a new toplevel:pulsar CI job, which performs a top-level build of just LAL and LALPulsar. LALPulsar is the only library (apart from LALApps) which has optional dependencies on other LAL libraries, and this job will test that LALPulsar is buildable with those dependencies disabled.

API Changes and Justification

Backwards Compatible Changes

• This change does not modify any class/function/struct/type definitions in a public C header file or any Python class/function definitions
• This change adds new classes/functions/structs/types to a public C header file or Python module

Backwards Incompatible Changes

• This change modifies an existing class/function/struct/type definition in a public C header file or Python module
• This change removes an existing class/function/struct/type from a public C header file or Python module

Review Status

cc @adam-mercer @evan-goetz