Clean up the makefiles

[tswilliams]

The structure of the makefiles - and the majority of their contents - has stayed constant since about 2012. However, 5 years later, the software now supports more OSes which motivates new build-tool functionality (e.g. possibility of disabling build of certain externals). Also over the past few years, various minor modifications have been made. Finally, a large fraction of the build rules for uHAL are duplicated across the different packages - these common rules & config could be moved into common files.

So the makefiles are overdue for some improvements and cleaning, which will be split up into several pull requests / issues that are referenced from this issue.

Non-exhaustive TODO/wish list:

1. Specify which 'extern' packages are built using environment variables

• Motivation: Different 'extern' packages need to be built on different operating systems, but ideally, the command for building the software should be the same on each OS (simply make for uHAL + ControlHub)
• Initial idea:
  • Include/exclude build of each of the 'extern' packages based on value of new environment variables BUILD_BOOST, BUILD_PUGIXML and BUILD_ERLANG
  • Set the default value of these variables based on the OS (i.e. CACTUS_OS env var)
  • ... but still allow user to override these default values (e.g. by running make BUILD_BOOST=1 on centos7)

2. Move duplicate rules/config from the several uHAL package makefiles into a common set of files

• E.g. the src -> obj, and obj -> lib rules, g++ flags, and RPM-related rules/config
• Can use similar work in SWATCH & MP7 as reference here

3. Review what g++ flags are used on each OS

• In order to have as strict checks as possible on the source code, while being wary of enabling warning messages that the source code will unavoidably produce.

[tswilliams]

All done. However, it would be useful to have a 2nd round of minor cleanup in the near future. Closing.