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

Remove empty string handling edge-cases

An empty string will rarely be useful, since the only thing that
can be done to it is appending a character with the current state
of the string API. Storing empty strings with a NULL storage pointer
creates unnecessary edge cases in any code handling strings.

The tables test no longer segfaults.

Test for empty table cells

Support $SOURCE_DATE_EPOCH for reproducible man pages

The environment variable SOURCE_DATE_EPOCH [0] is standardized and can
be used to produce reproducible output. Distributions like Debian will
set this variable before the build and scdoc should use it (instead of
the current date) for any timestamps within the man pages.

[0]: https://reproducible-builds.org/docs/source-date-epoch/

Don't have side effects in assert

Add .editorconfig

Update README.md

Update version to 1.5.2

Add '.' to characters permitted in man page titles

Update version to 1.5.1

Fix build of scdoc.5

Update version to 1.5.0

Add support for explicit line breaks

s/Generated content/Begin generated content/

Move premable comments from roff to C

Error if formatting continues to next paragraph

This prevents the inevitable "nested formatting" error from being too
far away from the source of the issue.

Show origin of nested formatting issues