Guard against CMAKE_GENERATOR in the environment

Problem Description

When building the SVT-AV1 contrib, I ran into an error because I had set CMAKE_GENERATOR=ninja in my environment (so that CMake generates ninja build files), but the Handbrake build script continued calling make, which obviously fails if no Makefile exists.

I've had this issue with SVT-AV1, but I assume the same issue exists for other CMake-based contribs.

I'm no Handbrake.git expert, but generally there are a couple ways to prevent this issue:

• set CMAKE_GENERATOR="" or unset CMAKE_GENERATOR when configuring CMake projects, to guard against the environment
• use cmake --build instead of make for CMake-based projects, so that the generator doesn't matter
• pass -G "Unix Makefiles" to CMake, to always generate Makefiles

Activity Log, Crash Log or any other details

user@host:/HandBrake/build$ make
make -C ./contrib/svt-av1/SVT-AV1-v1.7.0/build clean
make[1]: Entering directory '/HandBrake/build/contrib/svt-av1/SVT-AV1-v1.7.0/build'
make[1]: *** No rule to make target 'clean'.  Stop.
make[1]: Leaving directory '/HandBrake/build/contrib/svt-av1/SVT-AV1-v1.7.0/build'
make: [../contrib/svt-av1/module.rules:2: contrib/svt-av1/.stamp.svt-av1.build] Error 2 (ignored)
/usr/bin/rm -f ./contrib/svt-av1/.stamp.svt-av1.build
make -C ./contrib/svt-av1/SVT-AV1-v1.7.0/build
make[1]: Entering directory '/HandBrake/build/contrib/svt-av1/SVT-AV1-v1.7.0/build'
make[1]: *** No targets specified and no makefile found.  Stop.
make[1]: Leaving directory '/HandBrake/build/contrib/svt-av1/SVT-AV1-v1.7.0/build'
make: *** [../contrib/svt-av1/module.rules:3: contrib/svt-av1/.stamp.svt-av1.build] Error 2

What Operating System are you running?

Ubuntu 23.10

What version of HandBrake are you running?

1.7.3

Where did you download HandBrake from?

No response

Review contrib/svt-av1/module.rules and the CMake-based contrib build rules, then reproduce the failure with CMAKE_GENERATOR=ninja in the environment. Done means the affected contrib builds successfully without requiring a Makefile when that environment variable is set.