Trac #27970: Disable vpx support when building libgd for Cygwin

We don't use this functionality anyways.  If it happens to exist in the system
package that is also harmless.  It just happens that the system package for
libvpx is broken in some cases (Cygwin < 2.10).

build/pkgs/libgd/spkg-install

@@ -7,6 +7,14 @@ cd src
 
 export CFLAGS="-g $CFLAGS"
 
+if [ "$UNAME" = "CYGWIN" ]; then
+    # Compiling with vpx support creates a broken library in some cases
+    # because the vpx package itself is broken on some older Cygwin versions;
+    # we don't need this feature so safer to just disable
+    # https://trac.sagemath.org/ticket/27970
+    LIBGD_CONFIGURE="--without-vpx $LIBGD_CONFIGURE"
+fi
+
 # We explicitly disable X and fontconfig support, since (1) X is not a SAGE dependency,
 # and (2) the gd build fails on a lot of OS X PPC machines when X is enabled.
 sdh_configure --without-jpeg --without-xpm --without-x --without-fontconfig \