Sync third_party/nlbuild-autotools with upstream

This syncs third_party/nlbuild-autotools with the upstream rev 1.6.15

Author: samhansen

third_party/nlbuild-autotools/repo/CHANGES

@@ -1,3 +1,7 @@
+1.6.15 (2019-12-06)
+
+        * Remove redundant libtool m4 libraries.
+
 1.6.14 (2019-07-16)
 
         * Update MANIFEST to ensure that 'make dist' works correctly.

third_party/nlbuild-autotools/repo/MANIFEST

@@ -79,11 +79,6 @@ third_party/autoconf/m4/ax_cxx_compile_stdcxx_14.m4
 third_party/autoconf/m4/ax_jni_include_dir.m4
 third_party/autoconf/m4/ax_prog_doxygen.m4
 third_party/autoconf/m4/ax_pthread.m4
-third_party/autoconf/m4/libtool.m4
-third_party/autoconf/m4/ltoptions.m4
-third_party/autoconf/m4/ltsugar.m4
-third_party/autoconf/m4/ltversion.m4
-third_party/autoconf/m4/lt~obsolete.m4
 third_party/autoconf/m4/pkg.m4
 third_party/autoconf/missing
 third_party/autoconf/mkinstalldirs