audio/nyquist: add USE_CSTD=c99 to fix build on GCC-based systems:

  nyqsrc/sndseq.c:81: error: 'for' loop initial declaration used outside C99 mode

audio/nyquist/Makefile

@@ -10,6 +10,7 @@ COMMENT=	Language for sound synthesis and music composition (sound is broken) #
 LICENSE=	BSD3CLAUSE
 
 USES=		cmake zip
+USE_CSTD=	c99
 
 WRKSRC=		${WRKDIR}/${PORTNAME}