Add allow_abs_paths option

The existing error upon detecting use of an absolute include directory
path (within the source tree) is a well-intentioned but blunt tool that
unnecessarily prevents use of valid and safe absolute include dir paths.

Absolute paths can be constructed from build-relative absolute paths
(e.g. src_root = project_source_root(), ... src_root / 'dir/inc') and so
use consistent 'src_root'-relative path construction across multiple
meson.build files while also making each meson.build file easy to move
without breaking through use of pure relative paths.

Discussed with more context and rationale here -
mesonbuild#12244

Setting the option `allow_abs_paths=true` prevents this error check and
is obviously used at your own risk.

docs/markdown/Builtin-options.md

@@ -96,6 +96,7 @@ machine](#specifying-options-per-machine) section for details.
 | wrap_mode {default, nofallback,<br>nodownload, forcefallback, nopromote} | default | Wrap mode to use                   | no             | no                |
 | force_fallback_for                     | []            | Force fallback for those dependencies                          | no             | no                |
 | vsenv                                  | false         | Activate Visual Studio environment                             | no             | no                |
+| allow_abs_paths                        | false         | Suppress errors from use of absolute include paths             | no             | yes               |
 
 #### Details for `backend`
 

mesonbuild/coredata.py

@@ -1380,6 +1380,7 @@ def add_to_argparse(self, name: str, parser: argparse.ArgumentParser, help_suffi
     (OptionKey('wrap_mode'),       BuiltinOption(UserComboOption, 'Wrap mode', 'default', choices=['default', 'nofallback', 'nodownload', 'forcefallback', 'nopromote'])),
     (OptionKey('force_fallback_for'), BuiltinOption(UserArrayOption, 'Force fallback for those subprojects', [])),
     (OptionKey('vsenv'),           BuiltinOption(UserBooleanOption, 'Activate Visual Studio environment', False, readonly=True)),
+    (OptionKey('allow_abs_paths'), BuiltinOption(UserBooleanOption, 'Suppress errors from use of absolute include paths', False, yielding=False)),
 
     # Pkgconfig module
     (OptionKey('relocatable', module='pkgconfig'),

mesonbuild/interpreter/interpreter.py

@@ -2795,7 +2795,7 @@ def build_incdir_object(self, incdir_strings: T.List[str], is_system: bool = Fal
         absbase_build = os.path.join(build_root, self.subdir)
 
         for a in incdir_strings:
-            if path_is_in_root(Path(a), Path(src_root)):
+            if not self.coredata.options[OptionKey('allow_abs_paths')].value and path_is_in_root(Path(a), Path(src_root)):
                 raise InvalidArguments(textwrap.dedent('''\
                     Tried to form an absolute path to a dir in the source tree.
                     You should not do that but use relative paths instead, for

mesonbuild/utils/universal.py

@@ -2185,6 +2185,7 @@ class OptionType(enum.IntEnum):
     'pkg_config_path',
     'cmake_prefix_path',
     'vsenv',
+    'allow_abs_paths',
 }