[OF signal] Compilation errors due to mpi.h

[Juan Pablo Carbajal]

During installation of latest signal package there are errors due to
mpi.h not found. I guess this is another instance of badly defined
CPPFLAGS.

Is there a way we can solve this problem in a consistent way?
This involves improving configure.ac and PKG_ADD. If we can't find the
headers in a few reasonable places then we could prompt the user. Or
is there another way of doing this? (besides asking the user to export
the correct CPPFLAGS, which will put off many users).