[OSX] build fails

I'm trying to build the python wrapper for gdk on my mac (BigSur) with these steps:
```
$ mkdir gdk-python
$ python3 -m venv venv
$ source venv/bin/activate
$ pip install -r ./tools/requirements.txt
$ ./tools/build.sh --install $PWD/gdk-python --clang --python-version 3.9
```
but i get following error on the very last command (`./tools/build.sh ...`):
```
The Meson build system
Version: 0.58.0
Source dir: /gdk
Build dir: /gdk/build-clang
Build type: native build
Project name: gdk
Project version: 0.0.54

meson.build:1:0: ERROR: Compiler clang can not compile programs.

A full log can be found at /gdk/build-clang/meson-logs/meson-log.txt
```

And this is the content of the `meson-log.txt` log file:
```
Build started at 2022-06-27T16:48:48.830448
Main binary: /usr/local/opt/python@3.9/bin/python3.9
Build Options: -Dpython-version=3.8 -Dbuildtype=release -Ddefault_library=shared -Dwerror=True
Python system: Darwin
The Meson build system
Version: 0.62.2
Source dir: /gdk
Build dir: /gdk/build-clang
Build type: native build
Project name: gdk
Project version: 0.0.54
Sanity testing C compiler: clang
Is cross compiler: False.
Sanity check compiler command line: clang sanitycheckc.c -o sanitycheckc.exe -isysroot /Library/Developer/CommandLineTools/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk -mmacosx-version-min=10.13 -O3
Sanity check compile stdout:

-----
Sanity check compile stderr:
clang: warning: no such sysroot directory: '/Library/Developer/CommandLineTools/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk' [-Wmissing-sysroot]
ld: library not found for -lSystem
clang: error: linker command failed with exit code 1 (use -v to see invocation)

-----

meson.build:1:0: ERROR: Compiler clang can not compile programs.
```

How can I fix this?