Log Message:
Fixed a wrong warning for the HOMEPAGE variable

WARN: Makefile:9: Please use ${MASTER_SITE_GITHUB:=glmark2/glmark2}
      instead of "https://github.com/glmark2/glmark2".

This warning is wrong because HOMEPAGE is neither a plural variable nor
does it contain a "fetch URL".

To fix this, there is a new data type FetchURL, which has one more check
than the existing data type URL, namely the MASTER_SITES abbreviation.
The existing definitions in makevars.map have been updated accordingly
to the appropriate data types.

Note: This patch is not perfect, since ideally only variables of type
"List of FetchURL" should be suggested to be replaced with MASTER_SITES,
but since currently all these variables are actually "List of FetchURL"
and not simply "FetchURL", this should be fine.