Compile errors on Linux when using Qt5 built from source code

[Kazzz-S]

Dear @klayoutmatthias,

I'm trying to build KLayout version 0.27.13 on Linux Mint 20.3 Una.
For some reason (a bit trivial), I'd like to use a Qt5 built from source code.
Since the distribution provides tools using Qt 5.12.8, I chose qt-everywhere-src-5.12.8.tar.xz and could successfully install it under the standard directory, namely, /usr/local/Qt-5.12.8/.
While building KLayout using the /usr/local/Qt-5.12.8/bin/qmake tool, I got some compile errors.

To investigate the reasons, I have tested three different environments, as shown in the table below.

Step	Linux OS	Qt5 Tool Kit	KLayout Build Status	Log
1	Mint 20.3	5.12.8 built from source	compile errors	step1-qt5128-everywhere.zip
2	Mint 20.3	5.12.8 distributed binary pkg	success	step2-qt5128-MintUnaSystem.zip
3	Mint 20.3	5.15.7 built from source	compile errors	step3-qt5157-everywhere.zip

The apparent difference between Step 2 and other two is the presence of the -isystem flag in the g++ command line of Step 2.
However, I do not understand why and how qmake inserted this flag.
So far, I'm using the artifacts generated in Step 2.

I have also tested my older work environment:

Step	Linux OS	Qt5 Tool Kit	KLayout Build Status	Log
4	Mint 17.1	5.8.0 built from source	success	step4-qt580-everywhere.zip

Any suggestions to address this issue?
Thank you and best regards,
Kazzz-S

---

Edit-1

• In Qt 5.12.8, -isystem is set in mkspecs/common/gcc-base.conf as

QMAKE_CFLAGS_ISYSTEM        = -isystem

• A byproduct of this problem.