Added cloning for quiche and BoringSSL and modified the build
instructions so they work on a clean folder.
Closes #4208
## build
+Clone quiche and BoringSSL:
+
+ % git clone https://github.com/cloudflare/quiche
+ % cd quiche/
+ % mkdir deps
+ % cd deps
+ % git clone https://github.com/google/boringssl
+ % cd boringssl
+
Build BoringSSL (it needs to be built manually so it can be reused with curl):
- % mkdir -p quiche/deps/boringssl/build
- % cd quiche/deps/boringssl/build
+ % mkdir build
+ % cd build
% cmake -DCMAKE_POSITION_INDEPENDENT_CODE=on ..
% make -j`nproc`
% cd ..
% cd ..
% git clone https://github.com/curl/curl
+ % cd curl
% ./buildconf
% ./configure --with-ssl=$PWD/../quiche/deps/boringssl/.openssl --with-quiche=$PWD/../quiche --enable-debug
% make -j`nproc`