docs: add note that GCC 4.8 is needed to build gluon

The used nodejs 4.4.5 needs at least GCC 4.8 to be build
https://github.com/nodejs/node/blob/v4.x/BUILDING.md

Signed-off-by: Matthias Fritzsche <matthias@chemnitz.freifunk.net>
[Matthias Schiffer: moved information to the first section]

docs/releases/v2017.1.rst

@@ -16,6 +16,8 @@ contrast to packages that are common for all targets of the same architecture).
 Another change to the build system makes it necessary that the same *GLUON_RELEASE*
 value that is used to build the images is also set for ``make manifest``.
 
+GCC 4.8 or newer is now required to build Gluon.
+
 **Note: There is an issue in all Gluon versions before 2016.2.6 that will
 lead to x86 systems losing their configuration when upgrading to Gluon 2017.1!**
 Older Gluon versions should be upgraded to 2016.2.6 first before switching to