add -pthread helper

[kallisti5]

We added a -lpthread alias a while ago to help reduce the number of failed ports assuming you need -lpthread.

We are also missing "-pthread"

It could make sense to throw a warning in our GCC and continue onward (maybe ensuring -lroot?) vs erroring out.

~> gcc -lpthread test.c
~> # :-)

~> gcc -pthread test.c 
gcc: error: unrecognized command line option '-pthread'

[pulkomandy]

Fixed in btrev43191 and btrev43190