[WIP]libtool: build a shared library on Cygwin

Relates to esnet#514

Changed:
- enable libtool's Win32 DLL feature
- set libtool flags to force building a shared library

Build steps:
```
$ touch NEWS README AUTHORS ChangeLog
$ automake --add-missing --copy --foreign
$ autoreconf
$ ./configure
$ make -j8
```

Verification:
- build artifacts cre located under `src/.libs`
- there should be:
  - `cygiperf-0.dll` shared library
  - `iperf3.exe` executable binary linked to it
- `$ ldd src/.libs/iperf3.exe`

See:
- http://www.gnu.org/software/libtool/manual/html_node/LT_005fINIT.html
- https://www.sourceware.org/autobook/autobook/autobook_135.html
- https://www.sourceware.org/autobook/autobook/autobook_136.html
- https://www.sourceware.org/autobook/autobook/autobook_137.html
- http://stackoverflow.com/questions/26151995/autotools-make-doesnt-create-dll-under-cygwin
- https://autotools.io/libtool/windows.html
- https://cygwin.com/cygwin-ug-net/dll.html
- https://cygwin.com/problems.html
- http://stackoverflow.com/questions/25105573/compile-for-windows-using-mingw-w64-and-autotools

Signed-off-by: VirtualTam <virtualtam@flibidi.net>

configure.ac

@@ -45,7 +45,11 @@ AC_CANONICAL_HOST
 AC_PROG_CC
 AC_PROG_RANLIB
 AC_PROG_LN_S
+
+AC_LIBTOOL_WIN32_DLL
 AC_PROG_LIBTOOL
+AM_PROG_LIBTOOL
+LT_INIT
 
 # Add -Wall if we are using GCC.
 if test "x$GCC" = "xyes"; then

src/Makefile.am

@@ -5,6 +5,7 @@ include_HEADERS         = iperf_api.h # Defines the headers that get installed w
 
 
 # Specify the source files and flags for the iperf library
+libiperf_la_LDFLAGS     = -no-undefined -version-info 0:0:0
 libiperf_la_SOURCES     = \
                         cjson.c \
                         cjson.h \