Work-around for an issue which cropped up with the release of gcc-10. In their

Work-around for an issue which cropped up with the release of gcc-10. In their
infinite wisdom, they decided to build with -fno-common as default from now on,
breaking every piece of C code which used to declare common symbols in header
files, as was the convention since the dawn of time. We now have to duplicate
all declarations to an arbitrary source file, and change the header-file ones
to prefix them with extern.

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@1863 7f0cb862-5218-0410-a997-914c9d46530a