For performance, we only recompute .VARIABLES when (a) it's expanded
and (b) when its value will change from a previous expansion. To
determine (b) we were checking the number of entries in the hash
table which used to work until we started undefining entries: now if
you undefine and redefine the same number of entries in between
expanding .VARIABLES, it doesn't detect any change. Instead, keep
an increasing change number.
* variables.c: Add variable_changenum.
(define_variable_in_set, merge_variable_sets): Increment
variable_changenum if adding a new variable to the global set.
(undefine_variable_in_set): Increment variable_changenum if
undefining a variable from the global set.
(lookup_special_var): Test variable_changenum not the hash table.
* tests/scripts/variables/special: Test undefining variables.
Allows the user to reset the prefix character for introducing recipe lines
from the default (tab) to any other single character, and back again.
Also, reworked the manual to consistently use the word "recipe" to describe
the set of commands we use to update a target, instead of the various
phrases used in the past: "commands", "command lines", "command scripts",
etc.
