build system does not gracefully handle user-provided CXXFLAGS

[Apteryks]

Description

When the user provides CXXFLAGS, as in make CXXFLAGS=g3, the build breaks like:

ui/drivers/ui_qt.cpp:17:10: fatal error: QApplication: No such file or directory
   17 | #include <QApplication>

Expected behavior

Providing flags using user intended variables such as CXXFLAGS or CFLAGS should not remove flags that are needed for the build to succeed.

Actual behavior

The build breaks as shown in the description above.

Steps to reproduce the bug

1. Clone the source
2. Run the configure script
3. Attempt to build it with make CXXFLAGS=-g3

Bisect Results

Haven't bisected; happening with version 1.19.1.

Environment information

• OS: Guix System (GNU/Linux)
• Compiler: GCC 11

[zoltanvb]

Does it behave differently if you pass the variable in a different way?
CXXFLAGS=-g3 make

As in, there is an override behavior for the variables defined after the make command:
https://ftp.gnu.org/old-gnu/Manuals/make-3.79.1/html_chapter/make_9.html#SEC90