Use U+2018 instead of U+201B quote mark in compiler messages

This matches GCC's choice of Unicode quotation marks (i.e. U+2018 and U+2019)
and therefore looks more familiar on the console. This addresses #2507.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
(cherry picked from commit 018676c7f883886b388652c913c99a10d2591b0b)