Don't require a custom `.cargo/config` to build

[Diggsey]

The dependency issue with openssl (previously missing dependencies on user32-sys and gdi32-sys) has been fixed, and yet multirust still requires a custom cargo config to build.

For some reason, the dylib dependency from gdi32-sys does not get passed to the linker, despite it being present in the build-script output.

To reproduce this issue, install the openssl binaries exactly as the appveyor script does, and set these environment variables (assuming 64-bit):

DEP_OPENSSL_INCLUDE=C:\OpenSSL\openssl-1.0.2d-vs2015\include64
OPENSSL_INCLUDE_DIR=C:\OpenSSL\openssl-1.0.2d-vs2015\include64
OPENSSL_LIB_DIR=C:\OpenSSL\openssl-1.0.2d-vs2015\lib64
OPENSSL_LIBS=libeay32MT:ssleay32MT

Then do a clean cargo build. The expected result is a ton of warnings about missing pdb files, followed by the relevent error, a missing symbol from gdi32.lib.