cross-tools: fts has different types for cmp routine

fts has different types for its compare rotuine. Other systems, not
4.4BSD based, have a non-const version. Before we tested against
__GLIBC__, but now we test against __linux__ because that's Linux's API
and musl doesn't define __GLIBC__.

In addition, link against libftl on this platform since musl doesn't
include ftl routines in libc, but rather in libftl.

Co-authored-by:		Val Packett <val@packett.cool>
Sponsored by:		Netflix
Pull Request:		https://github.com/freebsd/freebsd-src/pull/1066
Reviewed by:		val_packett.cool
Differential Revision:	https://reviews.freebsd.org/D45349

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

lib/libpmc/pmu-events/jevents.c

@@ -1355,7 +1355,7 @@ int main(int argc, char *argv[])
 #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)

tools/build/mk/Makefile.boot

@@ -50,6 +50,10 @@ CFLAGS+=	-I${SRCTOP}/tools/build/cross-build/include/linux
 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

usr.sbin/kldxref/kldxref.c

@@ -745,7 +745,7 @@ usage(void)
 }
 
 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)