WebKit top of tree sources won't build in 10.9 w/ Xcode 6.0.1
https://bugs.webkit.org/show_bug.cgi?id=137053

Patch by David Gatwood. Reviewed, tweaked and landed by Alexey Proskuryakov.

Explain that to build from Xcode, one needs to build from command line once first.

Removed a Windows section that explained how to launch cygwin shell, which was
silly given that we then link to a long document explaining further steps.

* building/build.html:



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@174033 268f45cc-cd09-0410-ab3c-d52691b4dbfc