Makefile: Add MXE_DISABLE_{PROGRAMS,DOCS,CRUFT}.
tiff: don't install cruft

This gets rid of ca. 60 manpages and 60 HTML pages.

The 19 executables that were not built and installed before, continue
not being built/installed.
curl: don't install cruft

This gets rid of ca. 50 manpages and curl.exe.

We do keep installing curl-config, since that is used by other packages.
libzip: don't install cruft

This gets rid of ca. 60 manpages.
pcre: don't install cruft

This gets rid of ca. 100 manpages and various symlinks.

We do keep installing pcre-config, since that is used by other packages.
opus: don't install cruft

This gets rid of ca. 15 manpages, one info page and ca. 20 HTML pages.
libssh2: Use $(MXE_DISABLE_CRUFT).
libssh2: Disable building of examples.
Merge pull request #481 from uwehermann/no_cruft

Add MXE_DISABLE_{PROGRAMS,DOCS,CRUFT} and use it in a few packages