depends: Add a package for qt4.6. Linux uses it by default.

We're not ready to switch to a static qt5 for Linux yet due to missing plugin
support. This adds a recipe for building a shared qt4 that we build and link
against, but don't distribute.

make USE_LINUX_STATIC_QT5=1 can be used to build static qt5 as before.

depends/Makefile

@@ -6,6 +6,7 @@ SDK_PATH ?= $(BASEDIR)/SDKs
 NO_QT ?=
 NO_WALLET ?=
 NO_UPNP ?=
+USE_LINUX_STATIC_QT5 ?=
 FALLBACK_DOWNLOAD_PATH ?= https://bitcoincore.org/depends-sources
 
 BUILD = $(shell ./config.guess)