nullptr unknown on older GCC (GCC 5.4, Ubuntu 16 platform)

[GWRon]

[ 35%] Compiling:soloud_wavstream.cpp
[ 35%] Compiling:stb_vorbis.c
In file included from /home/ronny/BlitzMax/mod/audio.mod/soloud.mod/soloud/src/audiosource/wav/soloud_wavstream.cpp:32:0:
/home/ronny/BlitzMax/mod/audio.mod/soloud.mod/soloud/include/soloud_wavstream.h:54:23: error: ‘nullptr’ was not declared in this scope
    stb_vorbis *mOgg = nullptr;
                       ^
/home/ronny/BlitzMax/mod/audio.mod/soloud.mod/soloud/include/soloud_wavstream.h:55:20: error: ‘nullptr’ was not declared in this scope
    drflac *mFlac = nullptr;
                    ^
/home/ronny/BlitzMax/mod/audio.mod/soloud.mod/soloud/include/soloud_wavstream.h:56:18: error: ‘nullptr’ was not declared in this scope
    drmp3 *mMp3 = nullptr;
                  ^
/home/ronny/BlitzMax/mod/audio.mod/soloud.mod/soloud/include/soloud_wavstream.h:57:18: error: ‘nullptr’ was not declared in this scope
    drwav *mWav = nullptr;
                  ^
Build Error: failed to compile (256) /home/ronny/BlitzMax/mod/audio.mod/soloud.mod/soloud/src/audiosource/wav/soloud_wavstream.cpp

Seems we need to compile with c++-11 standards:

g++ -std=c++11

Another (less safe approach) is to replace nullptr with NULL

[GWRon]

Issue introduced here:
f625102

I meanwhile simply do this:
sed -i 's/= nullptr;/= NULL;/g' /home/ronny/BlitzMax/mod/audio.mod/soloud.mod/soloud/include/soloud_wavstream.h

to get rid of it but yeah ... dunno how much a NULL can have impacts there - at least it compiles.