Change our checks for MUMBLE_NO_PCH to use string comparisons
instead of numerical comparisons.
When MUMBLE_NO_PCH is not set, the numerical equivalent check
fails because nothing is on the left-hand side of the -eq operator.
Fixesmumble-voip/mumble#3314
This commit adds a Homebrew-based macOS build to our Travis CI build
matrix.
It also cleans up the exising build matrix such such that Linux builds
are built only on Linux hosts, and macOS builds are only built on macOS
hosts.
