Corrected bug in wrt_impulse.F

[arango]

The local variable Iinp was not assigned in routine wrt_impulse.F.

Also corrected a problem in build.bash when the macro definitions are commented out. We need to add a colon when testing if the macro expands to NULL:

We need to use:

if [ -n "${USE_MPIF90:+1}" ]; then
  ...
fi

indead of

if [ -n "${USE_MPIF90+1}" ]; then
  ...
fi

Many thanks to David Robertson for fixing this problem.