enable clang build

[CdeMills]

Hello,
following the release of debian 11 arm64 for Raspberry PI, I switched my RPI 3B and tried to compile piaware with clang and optimizations permitting to later increase the safety with the CFI flags.

I managed the build to succeed using this command line
env CC="clang" CXX=clang++ LDSHARED="clang -shared" EXTRA_LDFLAGS="-flto" DEB_BUILD_OPTIONS="hardening=+all optimize=+lto" dpkg-buildpackage -b --no-sign

with a few patches:

1. cx_Freeze
   generate extra_link_args from EXTRA_LDFLAGS
   cx_Freeze-6.8.3.txt

2. mlat_client

• debian/rules: switch to pybuild
• modes_message.c: clang spotted an error at line 94: use of || where | is expected
• setup.py: generate extra_link_args from EXTRA_LDFLAGS
  mlat_client.txt

3. tcllauncher
   This one was more difficult. The build process makes use of a LINK_CC variable, which may reflect the use of a framework called swiftc. The modification has to occur in configure.in
   tcllauncher.txt

The purpose:

1. ensure that "-flto" is used both during compile and link time
2. ensure that shared libs are build using "clang -shared" instead of "clang". The second expression fails as it tries to link to crt.o and complain about "main" being undefined.

I also disabled building the dump_978 module: clang complains about extensions (recursive namespace) with are defined only in C++-17

Regards

Pascal