Bug fix: tests didn't (always) return a valid status.

gdImageAALine_thickness.c didn't explicitly return 0 on success.

gdimageline_bug5.c was not an actual test at all.  It now confirms that the
tested operation (a line draw) alters the image it draws on.  (Not ideal
but better than what it did before.)
Added another Makefile.

This change adds a Makefile and bash script for building and testing
libgd on Windows with MinGW and MSYS.  This was written after I
concluded that it was easier and faster to just write a new Makefile
from scratch than to get autotools or CMake to play nicely with my
Windows installation.
Added beginnings of an updated manual

This changeset adds scripts and frontmatter for a user manual
for LibGD.  The manual is written using naturaldoc.  That is, the
actual manual (minus some front-matter taken from the manual for
version 2.0.36) is generated from specially-formatted comments in
the source code.

bootstrap.sh has been modified to also trigger generation of the
manual.
Added documention for gdImageCreate() and type gdImage.

This change adds naturaldoc comments for the above function and type.
Text was adapted from the 2.0.36 manual.

It also fixes the formatting of the comment for gdInterpolationMethod
so that naturaldoc will extract it.
Added .gitignore lines to exclude naturaldocs by-products.
Merge remote-tracking branch 'upstream/master'