allow proton to be built with add_subdirectory

Do not assume there is just one element in CMAKE_MODULE_PATH
Find files relative to the CMAKE_CURRENT_(SOURCE|BINARY)_DIR
PROTON-660: Fix openssl.c build on windows

*all files:
   Reorder includes so they are always from public to private

* platform.h:
   - replace strcasecmp with !pn_i_eq_nocase
   - replace strncasecmp with !pn_i_eq_n_nocase
   - include winsock.h on windows
   - undefine symbols 'ERROR' and 'small' that proton codebase uses.

The openssl is actually fixed by makeing sure
the winsock.h is included before openssl headers.