diff --git a/contrib/mandoc/config.h b/contrib/mandoc/config.h
--- a/contrib/mandoc/config.h
+++ b/contrib/mandoc/config.h
@@ -13,7 +13,7 @@
 #define HAVE_ENDIAN 0
 #define HAVE_ERR 1
 #define HAVE_FTS 1
-#if defined(__GLIBC__) || defined(__APPLE__)
+#if defined(__linux__) || defined(__APPLE__)
 #define HAVE_FTS_COMPARE_CONST 0
 #else
 #define HAVE_FTS_COMPARE_CONST 1
diff --git a/lib/libpmc/pmu-events/jevents.c b/lib/libpmc/pmu-events/jevents.c
--- a/lib/libpmc/pmu-events/jevents.c
+++ b/lib/libpmc/pmu-events/jevents.c
@@ -1355,7 +1355,7 @@
 #include <fts.h>
 
 static int
-#if defined(__GLIBC__) || defined(__APPLE__)
+#if defined(__linux__) || defined(__APPLE__)
 fts_compare(const FTSENT **a, const FTSENT **b)
 #else
 fts_compare(const FTSENT * const *a, const FTSENT * const *b)
diff --git a/tools/build/mk/Makefile.boot b/tools/build/mk/Makefile.boot
--- a/tools/build/mk/Makefile.boot
+++ b/tools/build/mk/Makefile.boot
@@ -50,6 +50,10 @@
 CFLAGS+=	-D_GNU_SOURCE=1
 # Needed for sem_init, etc. on Linux (used by usr.bin/sort)
 LDADD+=	-pthread
+.if exists(/usr/lib/libfts.so) || exists(/usr/lib/libfts.a) || exists(/lib/libfts.so) || exists(/lib/libfts.a)
+# Needed for fts_open, etc. on musl (used by usr.bin/grep)
+LDADD+=	-lfts
+.endif
 
 .elif ${.MAKE.OS} == "Darwin"
 CFLAGS+=	-D_DARWIN_C_SOURCE=1
diff --git a/usr.sbin/kldxref/kldxref.c b/usr.sbin/kldxref/kldxref.c
--- a/usr.sbin/kldxref/kldxref.c
+++ b/usr.sbin/kldxref/kldxref.c
@@ -745,7 +745,7 @@
 }
 
 static int
-#if defined(__GLIBC__) || defined(__APPLE__)
+#if defined(__linux__) || defined(__APPLE__)
 compare(const FTSENT **a, const FTSENT **b)
 #else
 compare(const FTSENT *const *a, const FTSENT *const *b)