build: Add --disable-adl-checks

Requested in https://github.com/veox/sgminer/issues/26

configure.ac needs an overhaul (see SDMINER_SDK).
build: reorder ADL checks.

If `--disable-adl-checks` is given, enable ADL immediately.
cross-compile: comment out sleep().

Reported as unneded:
https://github.com/veox/sgminer/issues/37

When is this true? Which IDEs, Windows/MinGW versions?
Add some notes about cross-compiling cgminer for Windows from Linux
doc: revert some formatting changes made by Drogean to windows-build.txt
Add clarification on Windows cross-build, out of tree is possible
doc: Simplify paths for cross-compiling instructions in windows-build.txt
doc: add note about --disable-adl-checks.

Should close https://github.com/veox/sgminer/issues/26
Merge branch 'windows-cross-compile'