Update required Qt version to 5.12

On Ubuntu 16.04 and 18.04 it is not possible to get Qt 5.12 from the
official repo. You will need to get it from
https://launchpad.net/~beineri/+archive/ubuntu/opt-qt-5.12.2-bionic
which places the files in /opt. This leads qmake to including the files
as non system headers, which makes them spam errors. Adding them as
-isystem headers first prevents this.

advancedSettings.pro

@@ -3,8 +3,8 @@ CONFIG   += c++1z
 
 DEFINES += ELPP_THREAD_SAFE ELPP_QT_LOGGING ELPP_NO_DEFAULT_LOG_FILE
 
-lessThan(QT_MAJOR_VERSION, 5): error("requires Qt 5.6 or higher")
-lessThan(QT_MINOR_VERSION, 6): error("requires Qt 5.6 or higher")
+lessThan(QT_MAJOR_VERSION, 5): error("requires Qt 5.12 or higher")
+lessThan(QT_MINOR_VERSION, 12): error("requires Qt 5.12 or higher")
 
 TARGET = AdvancedSettings
 TEMPLATE = app

build_scripts/qt/sources.pri

@@ -72,6 +72,9 @@ win32-clang-msvc {
 # Anything g++ or clang
 # In order to suppress warnings in third party headers
 *clang|*clang-g++|*clang-libc++|*g++* {
+    QMAKE_CXXFLAGS += -isystem $$[QT_INSTALL_HEADERS]
+    QMAKE_CXXFLAGS += -isystem $$[QT_INSTALL_HEADERS]/QtCore
+    QMAKE_CXXFLAGS += -isystem $$[QT_INSTALL_HEADERS]/QtGui
     QMAKE_CXXFLAGS += -isystem ../third-party/openvr/headers
     QMAKE_CXXFLAGS += -isystem ../third-party/easylogging++
 }