Use pkg-config for RPi EGL detection

The config test for eglfs-brcm would fail otherwise on modern Yocto-based
sysroots, as these have no /opt/vc, but have a proper egl.pc that has the
include path and libs.

Task-number: QTBUG-50838
Change-Id: Ibe8dcfbfe9aabe00fd8c040e6e41a4479ccb39bb
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

configure.json

@@ -709,7 +709,8 @@
         "eglfs-brcm": {
             "description": "Broadcom EGL (Rasberry Pi)",
             "type": "compile",
-            "test": "qpa/eglfs-brcm"
+            "test": "qpa/eglfs-brcm",
+            "pkg-config-args": "egl"
         },
         "eglfs-egldevice": {
             "description": "EGLDevice",

mkspecs/devices/linux-rpi3-g++/qmake.conf

@@ -1,7 +1,12 @@
-# qmake configuration for the Raspberry Pi 3 (32-bit) using the Broadcom graphics stack
+# qmake configuration for the Raspberry Pi 3 using the Broadcom graphics stack
 
 include(../common/linux_device_pre.conf)
 
+# For modern Yocto-based sysroots that have the proper include and lib
+# setup in egl.pc we will pull the necessary arguments from
+# there. However, to support Raspbian and older stuff that has /opt/vc
+# and possibly no pkg-config, have some static values as well:
+
 # I consider it a bug that this is required, but our EGL config.test _requires_ it
 QMAKE_LFLAGS           += -Wl,-rpath-link,$$[QT_SYSROOT]/opt/vc/lib