stella: fix bug when compiling with PPC altivec vectorization

PPC altivec vectorization triggers a bug when compiling with -std=c++11
because "bool" is redefined in altivec.h.

src/emucore/Event.hxx:112:23: error: cannot convert ‘bool’ to ‘__vector(4) __bool int’ in assignment
         myKeyTable[i] = false;
                       ^

Acording to a bug report in GCC [1], "You need to use -std=g++11 or
undefine bool after the include of altivec.h as context sensitive
keywords is not part of the C++11 standard".

So let's compile with -std=gnu++11 by default.

Tested on ARM, x86 and PPC.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58241#c3

Fixes:
http://autobuild.buildroot.net/results/0970d2c8e1787ceffc46b589522e53d52675e03c
http://autobuild.buildroot.net/results/ec1bc57675b6e53af0cd33d7b99cd2e3bf5d9d7e



Signed-off-by: Sergio Prado <sergio.prado@e-labworks.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>