my_timegm was introduced in 1900c7 to add a mktime fallback used in
4a6241, the subsequently removed in c53823. As a result of this code
shuffling, the jq mktime function (f_mktime) wound up using a fallback
even on systems where timegm is available, with incorrect results in
some time zones with DST, e.g.
% TZ=America/New_York jq -n '"2018-08-31T00:00:00Z"|fromdate|todate'
"2018-08-31T01:00:00Z"
This undoes 1900c7 and moves the workaround it added into the #else of
my_mktime.
Replace uses of the nonstandard $^ variable by explicitly specifying the
source filenames. This is the only obvious thing preventing the build
from running correctly on non-GNU make.
Muh Muhten