Allow for better integration of native builds using cmake

[trink]

Desired Use

Something like this would allow complete control of the build using only a cross platform cmake script:
build_unix = ["cmake $CARGO_ROOT", "make"]
bulid_windows = ["cmake -G "MinGW Makefiles" $CARGO_ROOT", "mingw32-make"]

1. support for platform specific build commands For native builds pass additional build variables in the environment #553
2. allow the variables to be used in the build command (would need a new $CARGO_ROOT variable)
3. set the current working directory to the $OUT_DIR when invoking the build commands to contain all cmake build files/directories there

Current Use

I am using cmake to check system dependencies and bootstrap/install the required c libraries but have to do something like the following:

[package]
build = "./build.sh"
...

#!/bin/bash
ROOT=$PWD
cd $OUT_DIR

cmake -DCMAKE_BUILD_TYPE=release $ROOT
CMAKE_RET=$?
if [ $CMAKE_RET -ne 0 ]; then
    exit $CMAKE_RET
fi

make
MAKE_RET=$?
if [ $MAKE_RET -ne 0 ]; then
    exit $MAKE_RET
fi

cp $OUT_DIR/lib/* $OUT_DIR
exit $?

1. save off the root directory and change into $OUT_DIR
2. cargo does not expose the profile being built so in this case release is always used For native builds pass additional build variables in the environment #553
3. cmake will perform the installs for me using $OUT_DIR as the prefix, however this puts my libs in $OUT_DIR/lib (they are managed via externalproject_add so I cannot easily alter the install hierarchy)
4. manually move the libs into $OUT_DIR because the -L cannot be altered Let Cargo packages modify the state of the build environment for other (non dependent) packages. #95 (comment) (this could also be done by the cmake wrapper script)

[alexcrichton]

While not fully implemented in Cargo itself, now that #792 is landed I'm going to close this. Crates can now have build dependencies (in the build-dependencies array) and something like a cmake-friendly crate can grow in the ecosystem over time, making the build script for a package quite small!