curl-config must be in PATH when building

The static version is building without any problem for me.
But when trying to use the produced libs in another project there are a lot of undefined when linking.
When looking in the libs you can see that the functions are present but with a "\_imp\_" in the start of the function names. The reason for this is that the functions has been declared with "__declspec(dllexport)" that must only be used when building dynamic libs. To get rid of this declaration "-DCURL_STATICLIB" must be used when compiling. It is the curl-config script that should take care of this.
But wrong curl-config is used (for me the Linux version is used). The curl-config produced by mxe when building curl must be used, but it is not in the path.
This change puts the mxe curl-config in the in the PATH. I have tested that the patch works.
But maybe it is better ways to do this. The curl package could do a soft link in a directory already in the PATH to  curl-config.