When a project is consuming unity as as subproject, unity headers,
static library and pkg config files are installed by `meson install`.
This can be fixed by using `meson install --skip-subprojects`, but this
must be repeated in all the distros packaging a project.
Fixed by disabling install when building as a subproject.
Fixes: #661
The following features from the CMake build have been implemented:
* Library version retrieved from unity.h.
* Extension support.
* Library, header, and package configuration file installation.
This commit is entirely based on existing work by Owen Torres.
