Improve documentation to compile with openPMD support on Summit

[lucafedeli88]

@NeilZaim noticed that following exactly the instructions in the manual to compile WarpX with openPMD support results in an error.

This is due to the fact that an old version of gcc is used to compile openPMD-api instead of the newer version loaded with module load gcc.

It is enough to do

export CXX=$(which g++)
export CC=$(which gcc)

to fix the issue.

This PR adds these two lines to the manual.

[ax3l]

Thanks a lot, I totally forgot that I had that in my profile! Yes, this is needed or otherwise the default system compiler - an ancient GCC 4.8.5 - will be picked up first.

Can we move this part at the end of the "module load" block? That's something one would usually put in a $HOME/warpx.profile file or similar and will be needed for #759 too :)