Define shared global variables as extern
It's too hard to create a merge request, thus I'm attaching the patch here only.
This issue had been identified during Fedora 32 mass rebuild, where the build fails with this error:
/usr/bin/ld: label.o:/builddir/build/BUILD/glabels-3.4.1/src/template-history.h:31: multiple definition of `gl_template_history'; glabels-batch.o:/builddir/build/BUILD/glabels-3.4.1/src/template-history.h:31: first defined here
/usr/bin/ld: label-text.o:/builddir/build/BUILD/glabels-3.4.1/src/font-history.h:31: multiple definition of `gl_font_history'; glabels-batch.o:/builddir/build/BUILD/glabels-3.4.1/src/font-history.h:31: first defined here
/usr/bin/ld: font-history.o:/builddir/build/BUILD/glabels-3.4.1/src/font-history.h:31: multiple definition of `gl_font_history'; glabels-batch.o:/builddir/build/BUILD/glabels-3.4.1/src/font-history.h:31: first defined here
/usr/bin/ld: template-history.o:/builddir/build/BUILD/glabels-3.4.1/src/template-history.h:31: multiple definition of `gl_template_history'; glabels-batch.o:/builddir/build/BUILD/glabels-3.4.1/src/template-history.h:31: first defined here
/usr/bin/ld: label.o:/builddir/build/BUILD/glabels-3.4.1/src/template-history.h:31: multiple definition of `gl_template_history'; glabels.o:/builddir/build/BUILD/glabels-3.4.1/src/template-history.h:31: first defined here
/usr/bin/ld: label-text.o:/builddir/build/BUILD/glabels-3.4.1/src/font-history.h:31: multiple definition of `gl_font_history'; glabels.o:/builddir/build/BUILD/glabels-3.4.1/src/font-history.h:31: first defined here
/usr/bin/ld: media-select.o:/builddir/build/BUILD/glabels-3.4.1/src/template-history.h:31: multiple definition of `gl_template_history'; glabels.o:/builddir/build/BUILD/glabels-3.4.1/src/template-history.h:31: first defined here
/usr/bin/ld: template-history.o:/builddir/build/BUILD/glabels-3.4.1/src/template-history.h:31: multiple definition of `gl_template_history'; glabels.o:/builddir/build/BUILD/glabels-3.4.1/src/template-history.h:31: first defined here
/usr/bin/ld: font-combo-menu.o:/builddir/build/BUILD/glabels-3.4.1/src/font-history.h:31: multiple definition of `gl_font_history'; glabels.o:/builddir/build/BUILD/glabels-3.4.1/src/font-history.h:31: first defined here
/usr/bin/ld: font-history.o:/builddir/build/BUILD/glabels-3.4.1/src/font-history.h:31: multiple definition of `gl_font_history'; glabels.o:/builddir/build/BUILD/glabels-3.4.1/src/font-history.h:31: first defined here