Python build: fail loudly if extension modules fail to build.

Otherwise, we can have a broken build:
2024-03-14T00:28:23.7126080Z /private/var/folders/1k/qq3pcbf12vb6vyblh81736p40000gn/T/python-build.20240314002650.3136/Python-3.11.7/Modules/_ctypes/callbacks.c:438:18: error: call to undeclared function 'ffi_prep_closure'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
2024-03-14T00:28:23.7172330Z         result = ffi_prep_closure(p->pcl_write, &p->cif, closure_fcn, p);
2024-03-14T00:28:23.7173100Z                  ^
2024-03-14T00:28:23.7173690Z 1 error generated.
2024-03-14T00:28:24.1751690Z
2024-03-14T00:28:24.1753070Z Failed to build these modules:
2024-03-14T00:28:24.1780620Z _ctypes

which causes problems later
(Xpra-org#27 (comment)).

gtk-osx-setup.sh

@@ -100,6 +100,7 @@ export PYTHON_CONFIGURE_OPTS="--enable-shared --with-system-ffi"
 #This really means pyenv's *python* version. It's poorly named but
 #it's defined by pyenv so it can't be changed.
 export PYENV_VERSION=$PYTHON_VERSION
+export PYTHONSTRICTEXTENSIONBUILD=1
 $PYENV install -v $PYENV_VERSION
 PIP="$PYENV_ROOT/shims/pip3"