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.
Removed 'unity_src' variable because Unity only one source file, changed library method to static only because there was some issues with using Unity as a shared library, cleaned sub meson.build script and fixed comment in sub source root.
