gcc-4.9 entered Debian testing first on May 3rd 2014: https://packages.qa.debian.org/g/gcc-4.9.html
. LMDE UP8 was released on February 14th 2014: http://blog.linuxmint.com/?p=2544
. Hence gcc-4.9 isn't included in LMDE till a next UP.
When will a next UP be? I don't know, but it might be worked on next as all the Linux Mint 17 releases have been completed this month.