build: Fix libsoup dependency detection
The current check fails in multiple ways:
- the variable 'soupapiversion' needs to be checked in the pkg-config file; using get_variable() will only check inside the dependency's build system when used as a subproject
- grilo does not have a variable named 'soupapiversion' anyway; it's called 'soup_api_version'
- this is far from being idiomatic Meson
We need to split the check into two:
- check for pkg-config variables when using a system dependency
- check for a variable in the internal dependency when using grilo as a subproject
In order to make this work we are going to need the following MR in Grilo: