unix/Makefile: Allow to override/omit pthread lib name.
unix/Makefile: Allow to override/omit pthread lib name.

For example, on Android, pthread functions are part of libc, so LIBPTHREAD
should be empty.

adafruit/circuitpython,pozetroninc/micropython,trezor/micropython,MrSurly/micropython,henriknelson/micropython,trezor/micropython,MrSurly/micropython,pramasoul/micropython,trezor/micropython,pfalcon/micropython,henriknelson/micropython,pramasoul/micropython,tobbad/micropython,adafruit/circuitpython,adafruit/circuitpython,bvernoux/micropython,henriknelson/micropython,bvernoux/micropython,pfalcon/micropython,MrSurly/micropython,MrSurly/micropython,henriknelson/micropython,kerneltask/micropython,kerneltask/micropython,selste/micropython,trezor/micropython,henriknelson/micropython,kerneltask/micropython,pramasoul/micropython,bvernoux/micropython,pozetroninc/micropython,pozetroninc/micropython,pramasoul/micropython,pfalcon/micropython,bvernoux/micropython,selste/micropython,adafruit/circuitpython,adafruit/circuitpython,selste/micropython,pfalcon/micropython,tobbad/micropython,selste/micropython,tobbad/micropython,adafruit/circuitpython,trezor/micropython,pfalcon/micropython,kerneltask/micropython,pramasoul/micropython,MrSurly/micropython,pozetroninc/micropython,tobbad/micropython,bvernoux/micropython,kerneltask/micropython,selste/micropython,pozetroninc/micropython,tobbad/micropython