In a from-scratch build, when running the configure script, which then
runs the set_soenv script, the default_images symlink (that is passed
to this function in the form of a Windows path, for some reason) does
not exist yet, and realpath fails anyway. So don't bother calling
realpath on Windows paths.

Although I don't know whether it then will cause a problem that the
cygpath -m call won't be able to expand the symlink as it doesn't
exist anyway. This is a mess. And if cygpath -m expands symlinks
anyway, why is the realpath needed at all?

As set_soenv.in used to always put BUILD_SOSL="true" in the
environment, we can delete the block completely that was to be
executed only if BUILD_SOSL was undefined.

No idea what it is, not used anywhere as far as I could see.

It was fixed as TRUE, so unconditionally change its uses according to
that.

Not used anywhere as far as I can see.

No idea what it is, doesn't occur anywhere else in the sources.

No idea in what circumstance one would want to use that.

And some baby steps for cross-compiling for iOS in particular.

At least odbcconfig is a GUI executable.

Apparently MSVC is clever enough to guess that a program with a main()
is intended to be linked as a CUI ("console") executable, and
correspondingly a program with a WinMain() as a GUI ("Windows")
executable. And our code seems to follow that convention.

Note that all this indeed is just convention; the OS of course has no
idea whether an executable's source code had a main function called
main() or WinMain(), if either at all. That is just a convention for C
and C++ programs. Whether an executable is for the console or
"Windows" subsystem is determined by a single flag in the executable
header, and this can even be changed with editbin. Nothing prevents a
console executable from opening windows and have a GUI, and nothing
forces a GUI executable to actually have a GUI.

It's called the .NET Framework, so don't talk about "frame".

Also align Usage help printout better.

 - fixes https://bugs.freedesktop.org/show_bug.cgi?id=37017

These are completely useless executables, and some virus scanners
thinks one of them is infected, so why even ship them...