Allow skipping installation of subprojects/dependencies

[EXtremeExploit]

very self explanatory:

Just for when declaring a dependency/external subproject, to have the ability to pass down an argument like skip_install and ignore all the installs inside that dependency
Something like: dep1 = dependency('mydependency', skip_install: true)

This is very useful in the case for dependencies which are most commonly satically linked but dont want any of the other docs/includes the project wants to install

I know --skip-subprojects is a thing, but there are ocassions where this is only desired with only one dependency/subproject, dont want to skip all of them, only specific ones