Fix prefix dependent option defaults (#6552)

* Extend test_prefix_dependent_defaults unit test to cover default case

Extend test_prefix_dependent_defaults unit test to cover the default
case, when the default prefix is '/usr/local'. (On Windows, the default
prefix is 'c:/')

* Restore adjusting option defaults depending on the default prefix

Restore adjusting option defaults, depending on the default prefix.
Droppped in d778a371

mesonbuild/coredata.py

@@ -501,7 +501,7 @@ class CoreData:
         # Create builtin options with default values
         self.builtins = {}
         for key, opt in builtin_options.items():
-            self.builtins[key] = opt.init_option()
+            self.builtins[key] = opt.init_option(key, default_prefix())
         self.builtins_per_machine = PerMachine({}, {})
         for for_machine in iter(MachineChoice):
             for key, opt in builtin_options_per_machine.items():
@@ -959,9 +959,9 @@ class BuiltinOption(T.Generic[_T, _U]):
         self.choices = choices
         self.yielding = yielding
 
-    def init_option(self) -> _U:
+    def init_option(self, name: str = 'prefix', prefix: str = '') -> _U:
         """Create an instance of opt_type and return it."""
-        keywords = {'yielding': self.yielding, 'value': self.default}
+        keywords = {'yielding': self.yielding, 'value': self.prefixed_default(name, prefix)}
         if self.choices:
             keywords['choices'] = self.choices
         return self.opt_type(self.description, **keywords)

run_unittests.py

@@ -1857,8 +1857,14 @@ class AllPlatformTests(BasePlatformTests):
             # N.B. We don't check 'libdir' as it's platform dependent, see
             # default_libdir():
         }
+
+        if mesonbuild.mesonlib.default_prefix() == '/usr/local':
+            expected[None] = expected['/usr/local']
+
         for prefix in expected:
-            args = ['--prefix', prefix]
+            args = []
+            if prefix:
+                args += ['--prefix', prefix]
             self.init(testdir, extra_args=args, default_args=False)
             opts = self.introspect('--buildoptions')
             for opt in opts: