export DLLs functions for MSVC with CMake

- Add 3 DEF files to export functions from Windows DLLs
  - Add gettimeofday to crypto/crypto.def (*1)
  - Remove gai_strerrorA from tls/tls.def (*1)
- Fix CMakeLists.txt to use DEF files as PRIVATE
- Change DLL import library file name since it duplicates with static library
- Ignore compiler warning C4267, and Edit CMAKE_C_FLAGS not to overwrite it (*1)
- Add USE_SHARED option to build openssl.exe with shared libraries (*1)

  (*1) recommended by @mcnameej

ssl/Makefile.am

@@ -4,6 +4,7 @@ lib_LTLIBRARIES = libssl.la
 
 EXTRA_DIST = VERSION
 EXTRA_DIST += CMakeLists.txt
+EXTRA_DIST += ssl.def
 
 libssl_la_LDFLAGS = -version-info @LIBSSL_VERSION@ -no-undefined
 libssl_la_LIBADD = $(abs_top_builddir)/crypto/libcrypto.la