diff --git a/lib/msun/Makefile b/lib/msun/Makefile
--- a/lib/msun/Makefile
+++ b/lib/msun/Makefile
@@ -97,17 +97,19 @@
 	s_sinpi.c s_sinpif.c \
 	s_tanpi.c s_tanpif.c
 
+# LIBCSRCDIR is defined lazily, so can't be used for exists() below
+LIBC_SRCTOP=${SRCTOP}/lib/libc
 # Location of fpmath.h and _fpmath.h
-.if exists(${LIBCSRCDIR}/${MACHINE_ARCH})
+.if exists(${LIBC_SRCTOP}/${MACHINE_ARCH})
 LIBC_ARCH=${MACHINE_ARCH}
 .else
 LIBC_ARCH=${MACHINE_CPUARCH}
 .endif
-CFLAGS+=	-I${.CURDIR}/src -I${LIBCSRCDIR}/include \
-	-I${LIBCSRCDIR}/${LIBC_ARCH}
+CFLAGS+=	-I${.CURDIR}/src -I${LIBC_SRCTOP}/include \
+		-I${LIBC_SRCTOP}/${LIBC_ARCH}
 SYM_MAPS+=	${.CURDIR}/Symbol.map
 
-VERSION_DEF=	${LIBCSRCDIR}/Versions.def
+VERSION_DEF=	${LIBC_SRCTOP}/Versions.def
 SYMBOL_MAPS=	${SYM_MAPS}
 
 # C99 long double functions