Dependence tracking broke implicit extern "C"

[Alexandre Oliva]

AIX 4.1's system headers are not C++ aware.  However, when compiling a
C Hello World program as C++, it fails to link because printf is
mangled.

It seems that some recent change in the dependence generators has
started treating the sysp flag as a boolean, instead of as a tri-state
variable, in which 0 indicates a user header; 1, a system header and
2, a system header implicitly extern "C".

This patch fixes it.  Tested on AIX 4.1 with the latest snapshot.  Ok
to install?