Fix cross-compilation and MSVC build with OpenSSL

[josusky]

When building for multiple platforms I encountered several issues. Fist of all MSVC (Windows) build with TLS support through OpenSSL requires additional library "crypt32". Secondly, detection of pthread library failed when a cross-compiler was used (target platform is not identical to source/build platform). The "FIND_PACKAGE(Threads)" works fine but "FIND_LIBRARY(...)" either fails or finds pthread library for the wrong platform. It seems to methat presence of "CMAKE_THREAD_LIBS_INIT" which is set by the "FIND_PACKAGE(Threads)" test is a sufficiently reliable to use it to set "VMIME_HAVE_PTHREAD" but I must admit that this is not really my cup of coffee :-)