Fix #1289, cmake script modernization

[jphickey]

Checklist (Please check before submitting)

• I reviewed the Contributing Guide.
• I signed and emailed the appropriate Contributor License Agreement to [email protected] and copied [email protected].

Describe the contribution
Use target properties to define interfaces and compiler definitions rather than referencing global variables or using "known" paths in other modules. This better aligns with current practices and creates a more robust build environment that is less dependent on specific path names existing in a given module.

This adds an "osal_public_api" interface target that contains the paths to the public API headers as its INTERFACE_INCLUDE_DIRECTORIES and any required compiler definitions as its INTERFACE_COMPILE_DEFINITIONS property. Applications should use this rather than referring to the "${OSAL_SOURCE_DIR}/src/os/inc" include path directly.

Fixes #1289

Testing performed
Build and confirm OSAL successfully builds on various platforms including Ubuntu 22.04 (new-ish CMake) and 18.04 (old-ish CMake).

Expected behavior changes
No impact to runtime behavior (no actual source code changes here)

Build script makes more use of CMake "INTERFACE" libraries to communicate compiler options, include paths, and definitions to dependencies, moving away from using global variables to perform this task. Notably, this permits the targets to be imported into another build more easily.

System(s) tested on
Ubuntu 18.04 and 22.04 (native, newer and older CMake)
RTEMS 4.11 and 5.0

Additional context
All changes should be backward compatible, in that the "magic variables" are still being honored as the current version of the CMake script had previously implemented:

• OSAL_API_INCLUDE_DIRECTORIES (input)
• OSAL_USER_C_FLAGS (input)
• UT_COVERAGE_COMPILE_FLAGS (output)
• UT_COVERAGE_LINK_FLAGS (output)

The preferred method of configuring these items going forward would be to get/set the corresponding properties on the osal_public_api interface target for include directories and compile options, and use the ut_coverage_compile and ut_coverage_link interface targets for coverage testing flags. Defining both methods for now allows for transition.

If the OSAL_OMIT_DEPRECATED flag is set, then the UT_COVERAGE flags will not be exported.

Contributor Info - All information REQUIRED for consideration of pull request
Joseph Hickey, Vantage Systems, Inc.

[chillfig]

Approved. Confirmation that testing on VxWorks is preferable before merging this pull request.

[jphickey]

Update - I did confirm the build using ppc-vxworks6.9 and all was fine, no issues observed.
Should be OK to merge!