Deal with strptime() on OS X and *BSD (fix #1415)

strptime() on OS X and *BSDs (reputedly) does not set tm_wday and
tm_yday unless corresponding %U and %j format specifiers were used.
That can be... surprising when one parsed year, month, and day anyways.
Glibc's strptime() conveniently sets tm_wday and tm_yday in those cases,
but OS X's does not, ignoring them completely.

This commit makes jq compute those where possible, though the day of
week computation may be wrong for dates before 1900-03-01 or after
2099-12-31.

tests/optional.test

@@ -2,9 +2,14 @@
 
 # strptime() is not available on mingw/WIN32
 [strptime("%Y-%m-%dT%H:%M:%SZ")|(.,mktime)]
-"2015-03-05T23:51:47Z"
 [[2015,2,5,23,51,47,4,63],1425599507]
 
+# Check day-of-week and day of year computations
+# (should trip an assert if this fails)
+last(range(365 * 199)|("1900-03-01T01:02:03Z"|strptime("%Y-%m-%dT%H:%M:%SZ")|mktime) + (86400 * .)|strftime("%Y-%m-%dT%H:%M:%SZ")|strptime("%Y-%m-%dT%H:%M:%SZ"))
+null
+[2099,0,10,1,2,3,6,9]
+
 # %e is not available on mingw/WIN32
 strftime("%A, %B %e, %Y")
 1435677542.822351