Compile option --enable-int-quality added to prefer accuracy over speed.

Changelog :

• Silence test for artsc-config if it is not there.

libmpg123:

• Finally provide position-independent code for x86 with assembly optimisations. The textrels are gone thanks to Won Kyu Park and Taihei Momma.
• Clarify some license language in files descending from the original MMX optimisation.
• Fix return value overflow check for MPG123_BUFFERFILL.
• Introduced mpg123_getformat2() to enable the FORMAT command for the generic control not stealing MPG123_NEW_FORMAT from the main playback loop. The sequence LOADPAUSED-FORMAT-PAUSE (play) is supposed to work now.
• Use of the i486 decoder is now discouraged more prominently, in configure output.

out123:

• Fix stupid crash with verbose mode and tone generation (print the string if the pointer is non-null, not if it is null).

libout123:

• More consistent error messages for dynamic and legacy (built-in) modules. Namely, you get a hint how if you choose a different module than the built-in ones for a static libout123.