Add the strict-prototype flag and correct all the offending prototypes

See merge request !536

Handle g++ as a compiler when -Wstrict-prototypes is an invalid option, this emits a warning which can become an error.

To add to the fun the standard test program uses 'int main()', so we need our own program to check things, also PACKAGE_URL is a macro re-define, also an error...

Fix C++ adversion to type demotion from long long to float

Shadowswift repairs

See merge request !551

# Conflicts:
#   src/gravity.h
#   tests/testFFT.c

Add Pressure-Energy (P-U) SPH

Closes #197

See merge request !540

Added minor changes to remove debugging checks and to remove the extra hydro loop during initialisation