Trying to restore compatibility with older compilers.

SWIG's -D command-line option is documented as "-D<symbol>", which seems
to mean that -DFOO=1 and -DFOO=0 are equivalent, and "#if (FOO==1)" is
never true.

I switched to "#ifdef PISM_USE_TR1" instead, and this seems to work.

I'll test with clang on OS X 10.9 tomorrow.