Implement executablePath for Solaris and make getBaseDir less platform-dependent

Use base-4.17 executablePath when possible, and fall back on getExecutablePath when it's not available. The sole reason why getBaseDir had #ifdefs was apparently that getExecutablePath wasn't reliable, and we could reduce the number of CPP conditionals by making use of executablePath instead.

I needed this change to make getBaseDir work correctly on Solaris.

• are either individually buildable or squashed
• have commit messages which describe what they do (referring to [Notes][notes] and tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a [Note][notes] and cross-reference it from the relevant places.
• add a testcase to the testsuite.
• replace this message with a description motivating your change