* src/main.c (define_makefiles): Simplify by using variable_buffer

* src/main.c (define_makefiles): Simplify by using variable_buffer

Toss the previous implementation which first computed a locally
allocated list of options, then used alloca() to create a buffer
of the right size and populate it.  Instead, use variable_buffer
to append to the string directly.  First add all single-letter
options without arguments, then add options with arguments.