[patch][cmake] Check if plugin-api.h provides LDPT_GET_SYMBOLS_V2

[Rafael Espíndola]

LDPT_GET_SYMBOLS_V2 was added back in 2011, but it looks like there
are still versions of plugin-api.h out there that don't have it.

This patch adds a cmake check so that we degrade gracefully with old versions.

I will write an autoconf version too, but I would like to get this one
reviewed first. In particular, I would like to know what you think
about adding gold-config.h.cmake. I like it because at least with
cmake it lets us keep all the changes in tools/gold.

Cheers,
Rafael
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