configure.ac: improve fts checks

The current code checks explicitly for musl and if so uses pkg-config to
find fts-standalone. This is suboptimal because musl may not be the only
libc to need a standalone fts implementation, and fts-standalone is an
old gentoo-specific name[1] so libabigail won't build on any other system
against musl.

Instead, use AC_SEARCH_LIBS to look for fts_open in libc then libfts. Do
this before the check for behaviour just in case someone has a standalone
and broken implementation.  I assume that the standalone fts is installed
on the standard search path, which should be a fair assumption.

As we're not using pkg-config anymore the required link flags are now in
LIBS, so remove all instances of FTS_CFLAGS and FTS_LIBS.

[1] https://gitweb.gentoo.org/repo/gentoo.git/commit/sys-libs/fts-standalone?id=a403fe6c61d1aaeecd58d521f3def2f6d3513449

	* configure.ac: Use AC_SEARCH_LIBS to look for fts_open in libc
	and in libfts if necessary.  Remove the use of FTS_LIBS which is
	now unnecessary.
	* src/Makefile.am: Do not use FTF_LIBS as it's now unnecessary.
	* tools/Makefile.am: Likewise.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Dodji Seketeli <dodji@redhat.com>

configure.ac

@@ -281,6 +281,10 @@ fi
 
 AC_SUBST(VISIBILITY_FLAGS)
 
+dnl glibc and BSD include fts into their libc, but musl does not so
+dnl check if we need to explicitly link to the standalone musl-fts.
+AC_SEARCH_LIBS([fts_open], [fts])
+
 dnl Older glibc had a broken fts that didn't work with Large File Systems.
 dnl We want the version that can handler LFS, but include workaround if we
 dnl get a bad one. Add define to CFLAGS (not AC_DEFINE it) since we need to
@@ -295,11 +299,6 @@ AS_IF([test "x$ac_cv_bad_fts" = "xyes"],
       [CFLAGS="$CFLAGS -DBAD_FTS=1",
        CXXFLAGS="$CXXFLAGS -DBAD_FTS=1"])
 
-dnl On musl, we need to find fts-standalone
-AS_CASE(
-  [${host_os}], [*-musl*], [
-    PKG_CHECK_MODULES([FTS], [fts-standalone])
-])
 
 dnl Check for dependency: libelf, libdw, libebl (elfutils)
 dnl Note that we need to use at least elfutils 0.159 but

src/Makefile.am

@@ -52,12 +52,12 @@ if BTF_READER
 libabigail_la_SOURCES += abg-btf-reader.cc
 endif
 
-libabigail_la_LIBADD = $(DEPS_LIBS) $(FTS_LIBS)
+libabigail_la_LIBADD = $(DEPS_LIBS)
 libabigail_la_LDFLAGS = -lpthread -Wl,--as-needed -no-undefined -version-info $(LIBABIGAIL_SO_CURRENT):$(LIBABIGAIL_SO_REVISION):$(LIBABIGAIL_SO_AGE)
 
 CUSTOM_MACROS = -DABIGAIL_ROOT_SYSTEM_LIBDIR=\"${libdir}\"
 
 AM_CPPFLAGS=\
-$(CUSTOM_MACROS) $(DEPS_CPPFLAGS) $(FTS_CFLAGS) \
+$(CUSTOM_MACROS) $(DEPS_CPPFLAGS) \
 -Wall -I$(abs_top_srcdir) -I$(abs_top_srcdir)/include \
 -I$(abs_top_builddir)/include -I$(abs_top_builddir)

tools/Makefile.am

@@ -41,8 +41,6 @@ abicompatdir = $(bindir)
 
 abipkgdiff_SOURCES = abipkgdiff.cc
 abipkgdiffdir = $(bindir)
-abipkgdiff_CPPFLAGS = $(FTS_CFLAGS)
-abipkgdiff_LDADD = $(FTS_LIBS)
 abipkgdiff_LDFLAGS = $(abs_top_builddir)/src/libabigail.la -pthread
 
 kmidiff_SOURCES = kmidiff.cc