Avoid C99 constructs

Avoid C99 constructs

Although gnulib requires C99, most of the code does compile with a
C90 compiler (perhaps with a lot of warnings).  Reinstate our C90
configuration test, and clean up a few C99 things that crept in.

* src/job.c (construct_command_argv_internal): Don't use loop-local
variables or C++ comments.
* src/read.c (eval_makefile): Don't use loop-local variables.