[Interest] Force use of QMAKE CXXFLAGS set through ENV Var

[Oliver Niebuhr]

Morning everyone.

OS: Windows 10
Compiler: MingW-64, 8.1.0
Qt: 5.12, latest Git (but not really important in that case)

What do I want:
A.) Build Qt5 from Source
A.1.) While forcing "g++" to use the flag "-pipe", without modifying 
"QMAKE.CONF" in the local Git Directory every time.

I tried:
Setting the following Environment Variables in the Windows CMD Shell:

* SET QMAKE="QMAKE_CFLAGS+=-pipe QMAKE_CXXFLAGS+=-pipe"
* SET QMAKE_CFLAGS+="-pipe"
* SET QMAKE_CXXFLAGS+="-pipe"
* SET G++="-pipe"
* SET QMAKE=-project "QMAKE_CXXFLAGS+=-std=c++11"

I tested with and without the --> "" <--

I also tried to use it as a Configure Parameter. For Example:
* %SystemDrive%\qt5srcgit\qt5\configure -verbose -platform win32-g++ 
-c++std c++1z -opensource -confirm-license QMAKE_CXXFLAGS+=-pipe
* %SystemDrive%\qt5srcgit\qt5\configure -verbose -platform win32-g++ 
-c++std c++1z -opensource -confirm-license -device-option 
QMAKE_CXXFLAGS+=-pipe

If anyone could point me to the right Direction, I would be very 
grateful! All the Stuff I have tried, is from the Internet and the Qt5 
Configure Script. And yes, I am aware that it makes not much difference 
using "-pipe" these Days. Its about reducing write cycles on my SSD. And 
also figuring out, how to optimize my Linux Scripts / Windows Batch 
Files further.

Thanks!
Oliver