black oil fluid system: keep the saturation pressure spline consistent

It was not updated if the set PvtoTable was used which then lead to
segfaults.
black oil fluid system: remove commented out density calculation code

I thought about this a bit and came to the conclusion that since the
composition of oil is already fixed in that context, only the pressure
dependance needs to be included in addition.
black oil fluid system: don't use the numerical value for the surface pressure directly

this is just a minor clean up which does not change anything...
ERT: link to the pthreads library

this is required to make the opm-core build succeed if ERT was build
with -DBUILD_SHARED_LIBS=OFF . (without it, I get errors like

    /home/and/src/ert/devel/libert_util/src/thread_pool_posix.c:328: error: undefined reference to 'pthread_create'
Build system: mark ERT as REQUIRED

de facto, this does not change anything because opm-parser is required
and it has a hard dependency on ERT. It is good style to declare all
prerequisites explicitly in each module, though. Thanks to [at]bska
for the catch.
Find-ewoms.cmake: update prerequisites