Fix build for cygwin

[jmp0x00]

In cygwin I've got the following errors during setup:

/usr/local/lib/librrd.a(librrd_la-rrd_graph.o): In function 'graph_cairo_setup':
/home/.../Downloads/rrdtool/rrdtool-1.7.2/src/rrd_graph.c:4327: undefined reference to 'cairo_surface_destroy'
/usr/local/lib/librrd.a(librrd_la-rrd_graph.o): In function 'bad_format_check':
/home/.../Downloads/rrdtool/rrdtool-1.7.2/src/rrd_graph.c:5570: undefined reference to 'g_regex_new'
...

So to prevent this I've added required libraries into setup script for cygwin platform.

rrdtool was built like this:

export LDFLAGS=-Wl,--no-undefined
export PYTHON=/bin/python3.6
./configure --disable-mmap --prefix=/usr/local --disable-tcl --disable-perl --disable-ruby
make
make install