[Qtwebengine] Passing CXXFLAGS to qtwebengine

[John Frankish]

> I can pass CXXFLAGS to Qt5 (and remove -g0 and -O2) without problems, but they are not passed to the majority of the qtwebengine build.
> 
> Where do compiler flags get passed to the webengine build?
> 
To add to this, qtwebengine compiles on a 64-bit machine, but has problems on a 32-bit machine.

Compiling qt5 32-bit without qtwebengine with -march=i486 -mtune=i686 -Os seems to work fine.

Compiling qtwebengine 32-bit completes (using 4gb ram and a swapfile), but seems to use 64-bit opcodes in error?

$ ldd /usr/local/lib/qt5/QtWebEngineProcess
Illegal instruction
$ file /usr/local/lib/qt5/QtWebEngineProcess
/usr/local/lib/qt5/QtWebEngineProcess: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux.so.2, for GNU/Linux 4.8.17, stripped

$ dmesg
...
traps: ld-linux.so.2[7982] trap invalid opcode ip:b7f07f56 sp:bf912390 error:0 in ld-2.28.so[b7efa000+15000]
traps: QtWebEngineProc[7983] trap invalid opcode ip:b7f6ff56 sp:bfad5120 error:0 in ld-2.28.so[b7f62000+15000]