Replace localtime with gmtime

The localtime() function gives the time relative to the users time.
This makes the generated manpage non reproducible. gmtime() gives
the time as UTC.

Allow continuation of table cells on the next line

Add pkg-config file, scdoc -v

Update version to 1.8.1

Use !isalnum instead of isspace for this_stuff

Update version to 1.8.0

Use a more robust approach for in-word-underscores

Update version to 1.7.1

Deal with bogus uninitialized warning (thx gcc)

Update version to 1.7.0

Ignore underscores in the middle_of_a_word

Update version to 1.6.1

Add CONVENTIONS to scdoc(5)

Don't use a GNU-extension for $SOURCE_DATE_EPOCH parsing

Code gratefully copied from the link below.

https://reproducible-builds.org/docs/source-date-epoch/#c

Fixup readme fixup

Fixup readme (thanks rvi 64!)

Redefine _XOPEN_SOURCE to value 600.

Fixes build errors on (Open|Free)BSD and Solaris.

Omit needless words, change description to UNIX/BSD style.

	Regardless of standards considerations, if there's any advice
	that needs to be hammered into man authors, it's to be concise
	and accurate, but not pedantic. As Will Strunk commanded,
	"Omit needless words."

	The most needless words of all are promotional. No man page
	should utter words like "powerful", "extraordinarily versatile",
	"user-friendly", or "has a wide range of options".

	-- Doug McIlroy[1]
	[1] https://lists.gnu.org/archive/html/groff/2018-11/msg00058.html

Update version to 1.6.0

Support empty table cells