build: prepare for FHS, where GISBASE is not the absolute truth

[nilason]

GISBASE is used as a means to retrieve paths to various resources in source code. With FHS (#3661) this is no longer possible. This adds a number of new environment variables to address this.

Currently this is addressed with following workaround:

grass/CMakeLists.txt

Lines 176 to 194 in 5e998d3

	if(WITH_FHS)
	file(CREATE_LINK ${OUTDIR}/${GRASS_INSTALL_DEMODIR}
	${RUNTIME_GISBASE}/demolocation SYMBOLIC)
	file(CREATE_LINK ${OUTDIR}/${GRASS_INSTALL_FONTSDIR} ${RUNTIME_GISBASE}/fonts
	SYMBOLIC)
	file(CREATE_LINK ${OUTDIR}/${GRASS_INSTALL_ETCDIR}/colors
	${RUNTIME_GISBASE}/etc/colors SYMBOLIC)
	file(CREATE_LINK ${OUTDIR}/${GRASS_INSTALL_ETCDIR}/colors.desc
	${RUNTIME_GISBASE}/etc/colors.desc SYMBOLIC)
	file(CREATE_LINK ${OUTDIR}/${GRASS_INSTALL_ETCDIR}/element_list
	${RUNTIME_GISBASE}/etc/element_list SYMBOLIC)
	file(CREATE_LINK ${OUTDIR}/${GRASS_INSTALL_ETCDIR}/renamed_options
	${RUNTIME_GISBASE}/etc/renamed_options SYMBOLIC)
	file(CREATE_LINK ${OUTDIR}/${GRASS_INSTALL_ETCDIR}/VERSIONNUMBER
	${RUNTIME_GISBASE}/etc/VERSIONNUMBER SYMBOLIC)
	file(MAKE_DIRECTORY "${GISBASE}/gui/wxpython")
	file(CREATE_LINK ${OUTDIR}/${GRASS_INSTALL_GUIDIR}/wxpython/xml
	${RUNTIME_GISBASE}/gui/wxpython/xml SYMBOLIC)
	endif()

This is work in progress.

[marisn]

Why GRASS_LOCALEDIR and GRASS_SHARE_DIR?