Deprecate the -no-qpa-platform-guard command line argument

There's no reason not to allow compiling Qt without any QPA
plugins. It's enough to clearly warn about it at the end
of configure instead of aborting the build.

Change-Id: I52ff681b68fcbfd9c5e28e555224812e6c441dc6
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

configure

@@ -766,7 +766,6 @@ CFG_SANITIZE_THREAD=no
 CFG_SANITIZE_MEMORY=no
 CFG_SANITIZE_UNDEFINED=no
 CFG_PCRE=auto
-QPA_PLATFORM_GUARD=yes
 CFG_STDCXX=auto
 CFG_DIRECTWRITE=no
 CFG_WERROR=auto
@@ -1908,7 +1907,7 @@ while [ "$#" -gt 0 ]; do
         ;;
     qpa-platform-guard)
         if [ "$VAL" = "yes" ] || [ "$VAL" = "no" ]; then
-            QPA_PLATFORM_GUARD="$VAL"
+            echo "WARNING: The [-no]-qpa-platform-guard argument is deprecated and has no effect."
         else
             UNKNOWN_OPT=yes
         fi
@@ -4566,10 +4565,6 @@ if [ "$CFG_SYSLOG" != "no" ]; then
     fi
 fi
 
-if [ "$CFG_GUI" = "no" ]; then
-    QPA_PLATFORM_GUARD=no
-fi
-
 # detect jpeg
 if [ "$CFG_LIBJPEG" = "auto" ]; then
     if compileTest unix/libjpeg "libjpeg"; then
@@ -5200,13 +5195,6 @@ if [ "$CFG_FONTCONFIG" != "no" ]; then
 
 fi
 
-# Save these for a check later
-ORIG_CFG_XCB="$CFG_XCB"
-ORIG_CFG_EGLFS="$CFG_EGLFS"
-ORIG_CFG_DIRECTFB="$CFG_DIRECTFB"
-ORIG_CFG_LINUXFB="$CFG_LINUXFB"
-ORIG_CFG_MIRCLIENT="$CFG_MIRCLIENT"
-
 if [ "$CFG_LIBUDEV" != "no" ]; then
     if [ -n "$PKG_CONFIG" ] && $PKG_CONFIG --exists libudev 2>/dev/null; then
         QMAKE_INCDIR_LIBUDEV=`$PKG_CONFIG --cflags-only-I libudev 2>/dev/null | sed -e 's,^-I,,g' -e 's, -I, ,g'`
@@ -5700,21 +5688,6 @@ if [ "$CFG_MIRCLIENT" = "yes" ]; then
     QT_CONFIG="$QT_CONFIG mirclient"
 fi
 
-if [ "$XPLATFORM_MAC" = "no" ] && [ "$XPLATFORM_MINGW" = "no" ] && [ "$XPLATFORM_QNX" = "no" ] && [ "$XPLATFORM_ANDROID" = "no" ] && [ "$XPLATFORM_HAIKU" = "no" ] && [ "$XPLATFORM_INTEGRITY" = "no" ]; then
-    if [ "$CFG_XCB" = "no" ] && [ "$CFG_EGLFS" = "no" ] && [ "$CFG_DIRECTFB" = "no" ] && [ "$CFG_LINUXFB" = "no" ] && [ "$CFG_MIRCLIENT" = "no" ]; then
-        if [ "$QPA_PLATFORM_GUARD" = "yes" ] &&
-            ( [ "$ORIG_CFG_XCB" = "auto" ] || [ "$ORIG_CFG_EGLFS" = "auto" ] || [ "$ORIG_CFG_DIRECTFB" = "auto" ] || [ "$ORIG_CFG_LINUXFB" = "auto" ] || [ "$ORIG_CFG_MIRCLIENT" = "auto" ] ); then
-        echo "No QPA platform plugin enabled!"
-        echo " If you really want to build without a QPA platform plugin you must pass"
-        echo " -no-qpa-platform-guard to configure. Doing this will"
-        echo " produce a Qt that can not run GUI applications."
-        echo " The dependencies needed for xcb to build are listed in"
-        echo " src/plugins/platforms/xcb/README"
-        exit 1
-    fi
-fi
-    fi
-
 # double-conversion support
 if [ "$CFG_DOUBLECONVERSION" = "no" ]; then
     if ! compileTest common/xlocalescanprint "XLocaleScanPrint"; then
@@ -7056,6 +7029,14 @@ if [ "$CFG_RELEASE_TOOLS" = "yes" -a \( "$CFG_DEBUG" = "no" -o "$CFG_DEBUG_RELEA
     echo "NOTE: -optimized-tools is not useful in -release mode."
     echo
 fi
+if [ "$CFG_GUI" = "yes" ] && [ "$XPLATFORM_MAC" = "no" ] && [ "$XPLATFORM_MINGW" = "no" ] && [ "$XPLATFORM_QNX" = "no" ] && [ "$XPLATFORM_ANDROID" = "no" ] && [ "$XPLATFORM_HAIKU" = "no" ] && [ "$XPLATFORM_INTEGRITY" = "no" ]; then
+    if [ "$CFG_XCB" = "no" ] && [ "$CFG_EGLFS" = "no" ] && [ "$CFG_DIRECTFB" = "no" ] && [ "$CFG_LINUXFB" = "no" ] && [ "$CFG_MIRCLIENT" = "no" ]; then
+        echo
+        echo "No QPA platform plugin enabled! This will"
+        echo "produce a Qt that cannot run GUI applications."
+        echo
+    fi
+fi
 
 exec 1>&3 3>&- # restore stdout
 cat $outpath/config.summary # display config feedback to user