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.
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