Build failure on Raspberry Pi 4 without including libatomic

[thurask]

Since meganz/MEGAcmd#451 also applies to MEGA SDK, rebroadcasting the issue for visibility. To summarize, building MEGAcmd git master on Raspberry Pi 4 running Raspbian 10 fails (undefined reference to `__atomic_load_8') without including libatomic in both MEGAcmd and MEGA SDK, which can be accomplished by adding an autoconf step to both projects to check if libatomic is needed.

[developerbedoya]

I set the environment variable LDFLAGS when running ./configure. That solved the problem in my Raspberry Pi 3, using Raspbian Buster:

LDFLAGS="-latomic" ./configure