qt5: Build in source directory to get private includes

Building Qt 5 outside the source directory, several variables are not
set, causing the Qt WebKit build to fail [1].

    set(Qt5Gui_PRIVATE_INCLUDE_DIRS "")

[1] https://bugreports.qt.io/browse/QTBUG-37417

qt5.be0

@@ -6,7 +6,8 @@ SRCURL[0]="https://download.qt.io/archive/qt/${PKGVERSION[2]}/${PKGVERSION}/sing
 
 PREFIX=/usr/local/qt5
 
-# build_in_sourcedir
+# https://bugreports.qt.io/browse/QTBUG-37417
+build_in_sourcedir
 
 # BEE_MAKEFLAGS=-j1