python dependency: verify that the python header is installed for sysconfig

We may or may not care that the library can be found. If link_libpython
is false, we allow it to be missing and still find the dependency.

(cherry picked from commit e22dd43)
[backported to apply pre-code move]

# Conflicts:
#	mesonbuild/dependencies/python.py

Author: eli-schwartz

mesonbuild/modules/python.py

@@ -152,6 +152,9 @@ def __init__(self, name: str, environment: 'Environment',
         else:
             self._find_libpy(installation, environment)
 
+        if not self.clib_compiler.has_header('Python.h', '', environment, extra_args=self.compile_args):
+            self.is_found = False
+
     def _find_libpy(self, python_holder: 'PythonInstallation', environment: 'Environment') -> None:
         if python_holder.is_pypy:
             if self.major_version == 3: