Declare globals as extern in headers, and define in source file

Otherwise, each source file that includes the header gets its own
definition, and according to the C standard (C99 6.9p5):

> If an identifier declared with external linkage is used in an
> expression (other than as part of the operand of a sizeof operator
> whose result is an integer constant), somewhere in the entire
> program there shall be exactly one external definition for the
> identifier

Most compilers use the .bss section for zero data, but if it uses
.data instead, or if -Wl,--warn-common is used, this will cause a
linking error.

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