(creating src/Makefile): Put code inside 2nd arg to AC_OUTPUT as it should

(creating src/Makefile): Put code inside 2nd arg to AC_OUTPUT as it should
be; hopefully noone will again see fit to gratuitously break this and not
make a change log entry.  Optimized sed processing of Makefile.in and cpp
output; now preserves comments previously removed from the cpp input.
Eliminated temp file for cpp output.
Generate -U switches to undefine all identifiers that appear in the
directory name ${srcdir}; pass these to cpp.