Added configure check for separate fts library to stout.

On some systems (e.g., those that use musl instead of glibc),
the fts(3) family of functions are not provided by libc.
Instead, a separate fts library must be installed. If this is
the case, "libfts" should be added to the list of libraries that
stout is linked against.

Review: https://reviews.apache.org/r/48245/

3rdparty/stout/configure.ac

@@ -200,6 +200,22 @@ AS_IF([test "x${ac_cv_env_CFLAGS_set}" = "x"],
 AS_IF([test "x${ac_cv_env_CXXFLAGS_set}" = "x"],
       [CXXFLAGS="$debug_flags $optimize_flags"])
 
+# Check if we need to do anything special to support using the fts_*
+# functions. On some systems (e.g., those that use musl instead of
+# glibc), FTS support requires linking against an additional library.
+AC_CHECK_HEADERS([fts.h], [], [AC_MSG_ERROR([cannot find fts.h
+-------------------------------------------------------------------
+Failed to find fts.h. If this header file is not provided by libc
+on this system (e.g., musl), a separate FTS package must be installed.
+-------------------------------------------------------------------
+])])
+
+AC_SEARCH_LIBS(fts_close, fts, [], [AC_MSG_ERROR([cannot find libfts
+-------------------------------------------------------------------
+Failed to find fts_close. If this function is not provided by libc
+on this system (e.g., musl), a separate FTS package must be installed.
+-------------------------------------------------------------------
+])])
 
 ###############################################################################
 # Miscellaneous checks related to (un)bundled software.