feat: include environment CFLAGS for make

[adrian5]

This allows the user to pass their own flags. I wanted to pass -march=native -mtune=native (the fzf binary runs locally by design, so why not), but the Makefile overrides CFLAGS.

With the change, one can easily use e.g. this in nvim:

build = "CFLAGS='-flag=foo -flag=bar' make"

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With cmake (…) -DCMAKE_C_FLAGS='-flag=foo -flag=bar' -DCMAKE_BUILD_TYPE=Release (…) should already work as-is.

[Conni2461]

thanks :)