src.opts: Introduce MK_SOUND

PR:		291853
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Reviewed by:	zarychtam_plan-b.pwste.edu.pl, markj
Differential Revision:	https://reviews.freebsd.org/D54456

share/mk/src.opts.mk

@@ -172,6 +172,7 @@ __DEFAULT_YES_OPTIONS = \
     SERVICESDB \
     SETUID_LOGIN \
     SHAREDOCS \
+    SOUND \
     SOURCELESS \
     SOURCELESS_HOST \
     SOURCELESS_UCODE \
@@ -500,6 +501,11 @@ MK_CLANG_FULL:= no
 MK_LLVM_COV:= no
 .endif
 
+# CUSE is needed only by virtual_oss, but virtual_oss is part of MK_SOUND.
+.if ${MK_CUSE} == "no"
+MK_SOUND:= no
+.endif
+
 .if ${MK_ASAN} == "yes"
 # In order to get sensible backtraces from ASAN we have to install
 # llvm-symbolizer as /usr/bin/addr2line instead of the elftoolchain version.