strongswan: add build dependency for python and explicitly use python3

strongswan: add build dependency for python and explicitly use python3

Before:
checking for python... /myworkingdir/build/staging_dir/host/bin/python
checking for python version... xcode-select: Failed to locate 'python', requesting installation of command line developer tools.
checking for python platform... xcode-select: Failed to locate 'python', requesting installation of command line developer tools.
checking for GNU default python prefix... ${prefix}
checking for GNU default python exec_prefix... ${exec_prefix}
checking for python script directory (pythondir)... xcode-select: Failed to locate 'python', requesting installation of command line developer tools.
checking for python extension module directory (pyexecdir)... xcode-select: Failed to locate 'python', requesting installation of command line developer tools.

After:
checking for python version... 3.11
checking for python platform... darwin
checking for GNU default python prefix... ${prefix}
checking for GNU default python exec_prefix... ${exec_prefix}
checking for python script directory (pythondir)... ${PYTHON_PREFIX}/lib/python3.11/site-packages
checking for python extension module directory (pyexecdir)... ${PYTHON_EXEC_PREFIX}/lib/python3.11/site-packages

Signed-off-by: Josef Schlehofer <[email protected]>