axel: update to v2.16.1

needs _DARWIN_C_SOURCE defined to compile
at present

net/axel/Portfile

@@ -3,7 +3,7 @@
 PortSystem          1.0
 PortGroup           github 1.0
 
-github.setup        axel-download-accelerator axel 2.15 v
+github.setup        axel-download-accelerator axel 2.16.1 v
 categories          net www
 platforms           darwin
 maintainers         nomaintainer
@@ -19,8 +19,14 @@ long_description    Axel does the same thing any other accelerator does: \
 github.tarball_from releases
 use_xz              yes
 
-checksums           rmd160  35d88c8f7cb855c11649b19a69506d0fc90e83cc \
-                    sha256  70daa27abca5a3f1abe4be833d8f64205ac422416f5bcf396856eb6dd0fe9194
+# https://github.com/axel-download-accelerator/axel/issues/136
+configure.cppflags-append -D_DARWIN_C_SOURCE
+
+checksums           rmd160  dd67b14c59d49caf42e50132078b38e64bace99f \
+                    sha256  d34094eb63eaebc989f96eabdda091dcfb28e840bb8d274a5f216775941f59e4
+
+depends_build-append \
+                    port:pkgconfig
 
 depends_lib         port:gettext \
                     path:lib/libssl.dylib:openssl