When erroring out due to invalid prefix, print the value

Helps with debugging what's going wrong

mesonbuild/coredata.py

@@ -62,7 +62,7 @@ class UserStringOption(UserOption):
         if not isinstance(value, str):
             raise MesonException('Value "%s" for string option "%s" is not a string.' % (str(newvalue), self.name))
         if self.name == 'prefix' and not os.path.isabs(value):
-            raise MesonException('Prefix option must be an absolute path.')
+            raise MesonException('Prefix option value \'{0}\' must be an absolute path.'.format(value))
         if self.name in ('libdir', 'bindir', 'includedir', 'datadir', 'mandir', 'localedir') \
             and os.path.isabs(value):
             raise MesonException('Option %s must not be an absolute path.' % self.name)

mesonbuild/mesonmain.py

@@ -82,7 +82,7 @@ class MesonApp():
     def __init__(self, dir1, dir2, script_file, handshake, options):
         (self.source_dir, self.build_dir) = self.validate_dirs(dir1, dir2, handshake)
         if not os.path.isabs(options.prefix):
-            raise RuntimeError('--prefix must be an absolute path.')
+            raise RuntimeError('--prefix value \'{0}\' must be an absolute path: '.format(options.prefix))
         if options.prefix.endswith('/') or options.prefix.endswith('\\'):
             options.prefix = options.prefix[:-1]
         self.meson_script_file = script_file