cmMakefile: return a backtrace

This allows backtraces to be fully controlled by the makefile rather
than externally (and makes changing how they are manipulated easier).
cmake: remove dummy backtraces for IssueMessage
genex: remove the need for backtraces

Rather than making dummy backtraces and passing them around, just make
backtraces optional.
backtrace: Convert to local paths in IssueMessage

This is the only place we care show the FilePath to the user, so defer
the expensive relative path calculation until here.
tests: update unused variable test expected output

The expected output is now a relative path, not a full path. Update the
pass/fail detection accordingly.
Merge topic 'dev/backtrace-performance' into next

3495ab0a tests: update unused variable test expected output
2a1b2d84 backtrace: Convert to local paths in IssueMessage
a0829205 genex: remove the need for backtraces
efc20569 cmake: remove dummy backtraces for IssueMessage
d46c650d cmMakefile: return a backtrace
FindOpenGL: Add missing quotes
Merge topic 'FindOpenGL-target' into next

3e2d6b4a FindOpenGL: Add missing quotes