Support for MSVC DLL

[jowr]

Prerequisites

• Checked the developer manual
• Checked that your issue isn't already filed: https://github.com/issues?utf8=✓&q=repo%3Alibtom%2Flibtomcrypt
• Checked that your issue isn't related to TomsFastMath's limitation that PK operations can by default only be done with max. 2048bit keys

Description

I have recently patched the 'makefile.msvc' to create a shared library on Windows using MSVC. Would you be interested in a PR to port these changes to the develop branch or are you planning to overhaul the build system anyway?

Version

My changes apply to v1.18.2