I had also received the same warnings when running make:
"INFO: Could not locate ffi libs and/or headers"
and "Failed to build these modules: _ctypes". Although Python could still be installed, the pip command was not installed, and installing it using get_pip.py just led to an error indicating that the module "_ctypes" was not found.

I can confirm that setting CPPFLAGS and LDFLAGS as well as PKG_CONFIG_PATH, as suggested my Mr. Levchenko, allowed me to build and install python 3.7.1 locally without root privileges with ctypes and pip included and working, using a libffi also compiled from source.