Tests: compiler cannot be overwritten

[FtZPetruska]

Description

The Makefiles present in each test folders do not allow the user to use a different compiler.

Steps to reproduce

Linux:

CC=$(which clang) make

macOS (gcc installed through homebrew):

CC=$(which gcc-11) make

Possible Solution

After testing around in Docker containers, I have noticed that simply removing the definition of CC inside the Makefiles works, because:

• If any compiler is installed, a cc binary will be added to your path which is a copy of the compiler. make will automatically use it. For example, on Ubuntu, if the user installs clang but not gcc they'd be able to compile it the tests.
• If the user defines CC themselves such as: CC=/path/to/cross/compiler make, make will use the cross compiler instead of the default.

Additionally, defining the CFLAGS with ?= instead of = allows the user to overwrite the default CFLAGS with their own. In case their compiler does not support one of the options.

Why

Although this may seem like nitpicking, this change would allow a user to cross-compile the tests without editing the Makefile and letting them ensure the headers behave as expected.

[empyreanx]

Thanks, I'll look into this when I have time. I'm pretty focused on other things right now