Remove warnings-as-errors by default

Compiling C++ on various different platforms with as many out of tree
dependencies as we have is volatile at best.
In order to prevent a third party warning from ruining the build for
people who don't care about third party warnings, we remove
warnings-as-errors by default.
They are still enabled by default on CI.

build_scripts/qt/compilers/clang.pri

@@ -48,7 +48,10 @@ QMAKE_CXXFLAGS += -Wmost
 
 QMAKE_CXXFLAGS += -Wpedantic
 
-QMAKE_CXXFLAGS += -Werror
+warnings_as_errors {
+    QMAKE_CXXFLAGS += -Werror
+}
+
 #All includes from the third-party directory will not warn.
 QMAKE_CXXFLAGS += --system-header-prefix=third-party
 QMAKE_CXXFLAGS += --system-header-prefix=qt

build_scripts/qt/compilers/gcc.pri

@@ -27,7 +27,9 @@ include(clang-gcc-common-switches.pri)
 
 QMAKE_CXXFLAGS += -Wpedantic
 
-QMAKE_CXXFLAGS += -Werror
+warnings_as_errors {
+    QMAKE_CXXFLAGS += -Werror
+}
 
 # GCC only switches
 QMAKE_CXXFLAGS += -Wduplicated-branches -Wduplicated-cond -Wlogical-op -Wrestrict -Wnull-dereference

build_scripts/qt/compilers/msvc.pri

@@ -1,7 +1,9 @@
 include(common-msvc.pri)
 
-# Warnings as errors
-QMAKE_CXXFLAGS += /WX
+warnings_as_errors {
+    # Warnings as errors
+    QMAKE_CXXFLAGS += /WX
+}
 
 # Enables Link Time Optimization
 QMAKE_CXXFLAGS += /GL