egl.prf: Append egl CFLAGS to QMAKE_C[XX]FLAGS

Projects including egl in their config (e.g. with CONFIG += egl) cannot get
egl CFLAGS without this.

Signed-off-by: Fatih Aşıcı <fatih.asici@gmail.com>

--- a/configure
+++ b/configure
@@ -4934,6 +4934,7 @@
         QMAKE_CFLAGS_EGL=`$PKG_CONFIG --cflags egl 2>/dev/null`
         QMakeVar set QMAKE_INCDIR_EGL "$QMAKE_INCDIR_EGL"
         QMakeVar set QMAKE_LIBS_EGL "$QMAKE_LIBS_EGL"
+        QMakeVar set QMAKE_CFLAGS_EGL "$QMAKE_CFLAGS_EGL"
     fi       # detect EGL support
     if compileTest qpa/egl "EGL" $QMAKE_CFLAGS_EGL $QMAKE_LIBS_EGL; then
         CFG_EGL=yes
--- a/mkspecs/features/egl.prf
+++ b/mkspecs/features/egl.prf
@@ -14,6 +14,8 @@
 } else {
     INCLUDEPATH += $$QMAKE_INCDIR_EGL
     LIBS_PRIVATE += $$QMAKE_LIBS_EGL
+    QMAKE_CFLAGS += $$QMAKE_CFLAGS_EGL
+    QMAKE_CXXFLAGS += $$QMAKE_CFLAGS_EGL
     LIBS += $$QMAKE_LFLAGS_EGL
     for(p, QMAKE_LIBDIR_EGL) {
         exists($$p):LIBS_PRIVATE += -L$$p