graphics/mesa-devel: unbreak standalone after e502b6d

llvm-config found: NO found '9.0.1' but need '>= 11.0.0'
Run-time dependency LLVM found: NO (tried cmake and config-tool)
Looking for a fallback subproject for the dependency llvm (modules: bitwriter, engine, mcdisassembler, mcjit, core, executionengine, scalaropts, transformutils, instcombine, amdgpu, native, bitreader, ipo, asmparser)

meson.build:1604:2: ERROR: Neither a subproject directory nor a llvm.wrap file was found.

graphics/mesa-devel/Makefile

@@ -94,7 +94,6 @@ anv_DESC=		Haswell and newer
 radv_DESC=		Southern Islands and newer (implies LLVM)
 
 COINST_DESC=		Allow co-installation with another Mesa port
-COINST_VARS=		LLVM_DEFAULT=11 # chase intel-compute-runtime
 COINST_MESON_ON=	-Degl-lib-suffix=${PKGNAMESUFFIX}
 COINST_PLIST_SUB=	SUFFIX=${PKGNAMESUFFIX}
 COINST_SUB_FILES=	libmap.conf
@@ -146,6 +145,13 @@ ZSTD_DESC=		Use ZSTD for shader cache
 ZSTD_LIB_DEPENDS=	libzstd.so:archivers/zstd
 ZSTD_MESON_ENABLED=	zstd
 
+.include <bsd.port.options.mk>
+
+# Ignore Mk/bsd.default-versions.mk but respect make.conf(5)
+.if !defined(DEFAULT_VERSIONS) || ! ${DEFAULT_VERSIONS:Mllvm*}
+LLVM_DEFAULT=	11 # chase intel-compute-runtime
+.endif
+
 post-extract:
 # Work around Meson lookup order e.g., llvm-config
 .if defined(BINARY_ALIAS)