fix(Build/Cmake): CMake now uses the Windows env vars (#3419)

* CMake wasn't able to find the enviroment variables which are noted in
the Wiki e.g MYSQL_INCLUDE_DIR or MYSQL_LIBRARY. Further I added
another variable for OpenSSL which is OPENSSL_ROOT_DIR.
This change allows .zip installations with a custom PATH.

src/cmake/macros/FindOpenSSL.cmake

@@ -28,6 +28,7 @@
 SET(_OPENSSL_ROOT_HINTS
   "[HKEY_LOCAL_MACHINE\\SOFTWARE\\Microsoft\\Windows\\CurrentVersion\\Uninstall\\OpenSSL (32-bit)_is1;Inno Setup: App Path]"
   "[HKEY_LOCAL_MACHINE\\SOFTWARE\\Microsoft\\Windows\\CurrentVersion\\Uninstall\\OpenSSL (64-bit)_is1;Inno Setup: App Path]"
+  "$ENV{OPENSSL_ROOT_DIR}"
   )
 
 IF(PLATFORM EQUAL 64)