Make Qt less smart about cross-toolchain detection and pkgconfig

When using the pkgconfig macros inside qmake
project files, qmake silently returns since it
detects being run inside a cross-toolchain and
pkgconfig normally doesn't work as expected in
such an environment.

However we patched pkgconfig to work inside
OpenWrt / LEDE (pkgconfig.real), so stop Qt
failing for qmake projects using pkgconfig.

frameworks/qt5base/patches/001-fix-pkgconfig-xcompile-detection.patch

@@ -8,4 +8,15 @@
 +#                return(false)
              }
  
+             pkgConfigSysrootDir = $$sysroot
+--- qtbase-opensource-src-5.8.0-rc/mkspecs/features/qt_functions.prf.orig	2017-01-19 17:52:11.741970700 +0100
++++ qtbase-opensource-src-5.8.0-rc/mkspecs/features/qt_functions.prf	2017-01-19 17:52:06.641997235 +0100
+@@ -272,7 +272,7 @@
+                         warning("Disabling pkg-config since PKG_CONFIG_SYSROOT_DIR is not set.")
+                         warning("Set this variable to your sysroot for pkg-config to function")
+                         warning("correctly when cross-compiling.")
+-                        return()
++#                        return()
+                     }
+ 
                      pkgConfigSysrootDir = $$sysroot