qmake use absolute paths instead of relative

qgroundcontrol.pro

@@ -7,14 +7,16 @@
 # License terms set in COPYING.md
 # -------------------------------------------------
 
+QMAKE_PROJECT_DEPTH = 0 # undocumented qmake flag to force absolute paths in make files
+
 exists($${OUT_PWD}/qgroundcontrol.pro) {
     error("You must use shadow build (e.g. mkdir build; cd build; qmake ../qgroundcontrol.pro).")
 }
 
 message(Qt version $$[QT_VERSION])
 
-!equals(QT_MAJOR_VERSION, 5) | !greaterThan(QT_MINOR_VERSION, 6) {
-    error("Unsupported Qt version, 5.7+ is required")
+!equals(QT_MAJOR_VERSION, 5) | !greaterThan(QT_MINOR_VERSION, 8) {
+    error("Unsupported Qt version, 5.9+ is required")
 }
 
 include(QGCCommon.pri)
@@ -207,8 +209,7 @@ contains(DEFINES, ENABLE_VERBOSE_OUTPUT) {
 } else:exists(user_config.pri):infile(user_config.pri, DEFINES, ENABLE_VERBOSE_OUTPUT) {
     message("Enable verbose compiler output (manual override from user_config.pri)")
 } else {
-CONFIG += \
-    silent
+    CONFIG += silent
 }
 
 QT += \