build: don't link to optional_mutex if it doesn't exist

Makefile.inc

@@ -14,11 +14,22 @@ IDIRS	:=	${IDIRS}
 # Default is no man pages
 MAN	=
 
+# The SUBDIR_DEPTH variable (defined above) initializes tracking the
+# subdirectory depth, but it's not fully set at this stage in the build
+# process.  Instead, we jump back to the shell and print the (finalized) value
+# of SUBDIR_DEPTH.  This avoids an infinite loop because we query the Makefile
+# (which does not include ../Makefile.inc), rather than Makefile.BSD (which
+# would end up including this file again).
+FINALIZED_SUBDIR_DEPTH	!=	${MAKE} -v SUBDIR_DEPTH
+
 .if !defined(NOLIBALL)
 # Link everything to liball.a, unless they specifically ask not to use it.
-# If appropriate, metabuild.sh will do:
+LIBALL	=	${SUBDIR_DEPTH}/liball/liball.a
+.if exists(${FINALIZED_SUBDIR_DEPTH}/liball/optional_mutex_normal)
+# If we have optional_mutex, add it. If appropriate, metabuild.sh will do:
 #     s/optional_mutex_normal/optional_mutex_pthread/g
-LIBALL	=	${SUBDIR_DEPTH}/liball/liball.a ${SUBDIR_DEPTH}/liball/optional_mutex_normal/liball_optional_mutex_normal.a
+LIBALL	+=	${SUBDIR_DEPTH}/liball/optional_mutex_normal/liball_optional_mutex_normal.a
+.endif
 LDADD	+=	${LIBALL}
 DPADD	+=	${LIBALL}
 .endif

release-tools/metabuild.sh

@@ -176,6 +176,9 @@ printf "RELATIVE_DIR=%s\n" "${D}" >> "${OUT}"
 if [ -n "$(${MAKEBSD} -v LIB)" ]; then
 	cat "${SUBDIR_DEPTH}/release-tools/Makefile.lib" >> "${OUT}"
 elif [ -n "$(${MAKEBSD} -v SRCS)" ]; then
+	# This *must* come after SUBDIR_DEPTH has been copied into
+	# the Makefile, because it depends on being able to run:
+	#     make -v SUBDIR_DEPTH
 	copyvar_LIBALL_optional_mutex
 	add_makefile_prog
 else