diff --git a/mk/buildlink3/bsd.builtin.mk b/mk/buildlink3/bsd.builtin.mk
index 7469253..8e97504 100644
--- a/mk/buildlink3/bsd.builtin.mk
+++ b/mk/buildlink3/bsd.builtin.mk
@@ -124,14 +124,29 @@
 
 .for _pkg_ in ${BUILTIN_PKG}
 #
-# Define PREFER.<pkg> to be either "pkgsrc" or "native" depending on
-# whether to prefer the pkgsrc or native versions of software that's
-# also part of the base system.  Its value is determined from the
-# user-settable values PREFER_PKGSRC and PREFER_NATIVE.  Preferences are
-# determined by the most specific instance of the package in either
-# PREFER_PKGSRC or PREFER_NATIVE.  If a package is specified in neither
-# or in both variables, then PREFER_PKGSRC has precedence over
-# PREFER_NATIVE.
+# Define PREFER.<pkg> to be either "pkgsrc" or "native" depending on whether
+# to prefer the pkgsrc or native version of a particular package.  Its value
+# is determined by a number of variables described below.
+#
+# PREFER.<pkg>
+#	This variable has the highest precedence, and should generally only be
+#	used by packages when it is known that a specific version should always
+#	be used or avoided.
+#
+# PREFER_PKGSRC
+# PREFER_NATIVE
+#	These user-settable variables either take "yes" or a list of package
+#	names.  The "yes" form has a lower precedence than a specific package,
+#	so the most common case is to set one variable to "yes" and then override
+#	specific packages in the other variable.
+#
+#	If a package is specified in neither or in both variables, then the
+#	PREFER_PKGSRC setting takes precedence.
+#
+# _OPSYS_PREFER.<pkg>
+#	This variable should only be set in mk/platform files, and provides a
+#	per-platform default that takes precedence over "yes" in PREFER_* but is
+#	overridden by a more specific package name match.
 #
 _BLTN_PREFER.${_pkg_}=	pkgsrc
 .  if !empty(PREFER_NATIVE:M[yY][eE][sS])
@@ -140,6 +155,9 @@ _BLTN_PREFER.${_pkg_}=	native
 .  if !empty(PREFER_PKGSRC:M[yY][eE][sS])
 _BLTN_PREFER.${_pkg_}=	pkgsrc
 .  endif
+.  if defined(_OPSYS_PREFER.${_pkg_})
+_BLTN_PREFER.${_pkg_}=	${_OPSYS_PREFER.${_pkg_}}
+.  endif
 .  if !empty(PREFER_NATIVE:M${_pkg_})
 _BLTN_PREFER.${_pkg_}=	native
 .  endif
diff --git a/mk/platform/Darwin.mk b/mk/platform/Darwin.mk
index 405fb38..dab8613 100644
--- a/mk/platform/Darwin.mk
+++ b/mk/platform/Darwin.mk
@@ -96,8 +96,8 @@ _USE_RPATH=		no	# don't add rpath to LDFLAGS
 KRB5_DEFAULT?=		mit-krb5
 
 # Builtin defaults which make sense for this platform.
-PREFER.linux-pam?=	native
-PREFER.mit-krb5?=	native
+_OPSYS_PREFER.linux-pam?=	native
+_OPSYS_PREFER.mit-krb5?=	native
 
 # flags passed to the linker to extract all symbols from static archives.
 # this is GNU ld.
diff --git a/mk/platform/HPUX.mk b/mk/platform/HPUX.mk
index 04dc6ce..57ea214 100644
--- a/mk/platform/HPUX.mk
+++ b/mk/platform/HPUX.mk
@@ -70,8 +70,8 @@ KRB5_DEFAULT?=		mit-krb5
 
 # Builtin openssl and zlib have issues. Prefer pkgsrc's until we know which
 # versions work.
-PREFER.openssl?=	pkgsrc
-PREFER.zlib?=		pkgsrc
+_OPSYS_PREFER.openssl?=	pkgsrc
+_OPSYS_PREFER.zlib?=	pkgsrc
 
 _STRIPFLAG_CC?=		${_INSTALL_UNSTRIPPED:D:U}	# cc(1) option to strip
 _STRIPFLAG_INSTALL?=	${_INSTALL_UNSTRIPPED:D:U}	# install(1) option to strip
diff --git a/mk/platform/QNX.mk b/mk/platform/QNX.mk
index dd2b557..48b7bca 100644
--- a/mk/platform/QNX.mk
+++ b/mk/platform/QNX.mk
@@ -100,7 +100,7 @@ GAMEDIRMODE=		0775
 
 PAM_DEFAULT?=		openpam
 
-PREFER.zlib?=		pkgsrc
-PREFER.bzip2?=		pkgsrc
-PREFER.curses?=		pkgsrc
-PREFER.termcap?=	pkgsrc
+_OPSYS_PREFER.zlib?=	pkgsrc
+_OPSYS_PREFER.bzip2?=	pkgsrc
+_OPSYS_PREFER.curses?=	pkgsrc
+_OPSYS_PREFER.termcap?=	pkgsrc
diff --git a/mk/platform/SunOS.mk b/mk/platform/SunOS.mk
index ff494b9..928ce83 100644
--- a/mk/platform/SunOS.mk
+++ b/mk/platform/SunOS.mk
@@ -41,10 +41,10 @@ MOTIF_TYPE_DEFAULT?=	motif
 KRB5_DEFAULT?=		mit-krb5
 
 # Builtin defaults which make sense for this platform.
-PREFER.libexecinfo?=	native
-PREFER.mit-krb5?=	native
-PREFER.openssl?=	pkgsrc
-PREFER.solaris-pam?=	native
+_OPSYS_PREFER.libexecinfo?=	native
+_OPSYS_PREFER.mit-krb5?=	native
+_OPSYS_PREFER.openssl?=		pkgsrc
+_OPSYS_PREFER.solaris-pam?=	native
 
 _OPSYS_EMULDIR.solaris=		# empty
 _OPSYS_EMULDIR.solaris32=	# empty