global vars and symbol visibility for mips32/elf

[Jim Wilson]

	The case which causes trouble is when a program uses a common variable
	(an uninitialized variable in C; most languages, including C++, have
	no notion of common variables).

ANSI/ISO C also does not have common variables.  A program with multiple
external definitions of a variable has undefined behaviour.  However, C
compilers that make this work by treating uninitialized variables as
common are so prevalent that most people aren't aware of this.  This
particular feature is mentioned in the `Common Extensions' appendix, in
the section `Multiple External Definitions'.

This also means that, technically, if you want to make sure that other
ANSI/ISO C compilers can compile your code, you must use -fno-common
in addition to -ansi and -pedantic.

Jim