#210 Compilation warnings and errors

[Darren Salt]

linux/i_sound.cpp: In function ‘bool
I_StartupSound(void*)’:
linux/i_sound.cpp:51: error: invalid conversion from
‘const ALubyte*’ to ‘const ALchar*’
linux/i_sound.cpp:51: error: initializing argument
1 of ‘ALCdevice* alcOpenDevice(const ALchar*)’

The attached patch fixes this (and also gets rid of
some warnings, some of which may be gcc4 only).

[Darren Salt]

Fix type-conversion error and some warnings