Generic overrider functionality

There are a ton of niggly one-off things people want to do which are reasonable to do but tricky and laborious to do by editing build definitions. Some edits can not even be done because they would touch subprojects. Examples of these include:

• changing the build type of some target to be shared or static
• change optimization level of one target (because, for example, compiler bugs)
• install or not install files from a given target or an entire subproject
• etc

All of these should be doable without changing the build definition files. One solution would be to add a Meson switch to read and use a file with these definitions.