--- a/lib/libc/Makefile
+++ b/lib/libc/Makefile
@@ -194,7 +194,7 @@ SUBDIR.${MK_TESTS}+= tests
 # recording a build dependency
 CFLAGS+= -I${SRCTOP}/lib/libutil
 # Same issue with libm
-MSUN_ARCH_SUBDIR != ${MAKE} -B -C ${SRCTOP}/lib/msun -V ARCH_SUBDIR
+MSUN_ARCH_SUBDIR = ${MACHINE_CPUARCH:S/i386/i387/}
 # unfortunately msun/src contains both private and public headers
 CFLAGS+= -I${SRCTOP}/lib/msun/${MSUN_ARCH_SUBDIR}
 .if ${MACHINE_CPUARCH} == "i386" || ${MACHINE_CPUARCH} == "amd64"