cmake --version  # ensure that you have >= 2.8.8

mkdir build
cd build
cmake ..
make

If you see an error about "Cannot find source file: OBJECT" then your version of CMake is too old.

Note that the default build flags in the top-leve CMakeLists.txt are for
debugging - optimisation isn't enabled.

If you'll be building a lot, then installing Ninja[1] is highly recommended.
Wipe out the build directory and recreate it, but using:

cmake -GNinja ..
ninja

If you want to cross-compile then there are example toolchain files for 32-bit
Intel and ARM in util/. Wipe out the build directory, recreate it and run cmake
like this:

cmake -DCMAKE_TOOLCHAIN_FILE=../util/arm-toolchain.cmake -GNinja ..

If you want to build as a shared library you need to tweak the STATIC tags in
the CMakeLists.txts and also define BORINGSSL_SHARED_LIBRARY and
BORINGSSL_IMPLEMENTATION. On Windows, where functions need to be tagged with
"dllimport" when coming from a shared library, you need just
BORINGSSL_SHARED_LIBRARY defined in the code which #includes the BoringSSL
headers.

[1] http://martine.github.io/ninja/