Merge pull request mxe#1659 from tonytheodore/gcc5-fix

Cleanups and fixes after gcc5 update

Merge pull request mxe#1538 from LuaAndC/build-pkg-recommends

build-pkg: system requirements are recommendations not dependencies

Merge pull request mxe#1071 from LuaAndC/build-pkg-dummy-for-requirem…

…ents

build-pkg: fix .deb installation

build-pkg: pkgs with broken deps are also broken

Bad behaviour prior to this commit:

1. The package is broken: pango
2. Package sdl_pango depends on broken pango
3. When t4k_common (which depends on sdl_pango) is built,
    pango is tried to build again.

This issue was caused by the error fixed in this commit:
if a package A depends on a broken package B,
then the package A is also BROKEN.
So if a package C depends on package A, then all of them
are broken:

(*)B <- A <- C

B is not "rebuilt" for C.

update 2.23 release date

Merge all well-tested changes into the stable branch

update release notes

fix the download URL mentioned in the tutorial

update release notes

Release 2.18