Allow libbson and libmongoc library names to be changed externally

[eldruin]

Internally we rename the libraries we integrate as to avoid interference with the library versions already available on the target systems by giving them a prefix.
This PR allows users to specify, for example:

-DBSON_OUTPUT_BASENAME:STRING=MYORG_bson
-DMONGOC_OUTPUT_BASENAME:STRING=MYORG_mongoc

I used BASENAME as the flavor and/or version are appended later.

Alternatively, it would be also fine for us to add a prefix variable like OUTPUT_NAME_PREFIX or so but I was unable to find a good name that cannot be confused the target PREFIX property (e.g. lib)

[rcsanchez97]

The changes look right to me, except for two very minor style issues. I've started the Evergreen patch build, so once the Travis and Evergreen builds complete, assuming no issues, I'll follow-up again with an approval.

[rcsanchez97]

@kevinAlbs Do these options to you like they should be documented in installing.rst? We don't document every build option it seems and there isn't a place where these naturally fall in that document, so it would need a new section. Also, do you have any reservations regarding this change?

[rcsanchez97]

Some build tasks are failing in Evergreen. A few I immediately recognize as known issues that are definitely not related to this change. I will need to confirm the others, but I honestly do not expect that any of the failures will be attributable to this change.

[kevinAlbs]

Thanks for the contribution @eldruin!