Improvemed patches to address travis and appveyor build errors

Reduced WINVER and _WIN32_WINNT to 0x0501 to be less restrictive

Refined CMakeLists.txt to allow for normal Windows and MinGW-specific OpenSSL certificate generation

Simplified include path to gettimeofday.h

Removed unnecessary list(APPEND LWS_LIBRARIES zlib_internal) export

Added back #include <windows.h> to gettimeofday.c to fix build for normal Windows

Made sure that pollfd gets defined on libwebsockets side when _WIN32_WINNT < 0x0600

Made sure that WINVER and _WIN32_WINNT don't get overridden by libwebsockets headers when already set to something greater than 0x0501

Added missing declaration of WSAPoll function for WINVER < 0x0600 in libwebsockets.h, eliminated invalid usages of pollfd instead of libwebsocket_pollfd in test-server.c

Cleaned up duplicate content in gettimeofday.c, removed header inclusions from gettimeofday.h and fixed include order in test-echo.c, test-ping.c and test-server.c to enable build with normal Windows and MinGW

Re-enabled debug_level in test-echo.c and made sure that the call to lws_set_log_level() is also active under Windows (just like in test-server.c); replaced all WIN32 occurrences by _WIN32 in test-echo.c, test-ping.c, and test-server.c

Removed build-msys.sh and added new section about how to build libwebsockets using MinGW to README.build.md