Windows : use meaningful DEFAULT_MYSQL_HOME - base directory

for the default installation.

It is now defined as "C:/Program Files/MariaDB ${MYSQL_BASE_VERSION}"
which is where installer indeed puts it by default.

It still does not cover every case -32bit installer on 64 bit Windows would put installation
root under  "C:/Program Files (x86)", but better than the path used
previously C:/MariaDB${MYSQL_BASE_VERSION}, which was never correct.

CMakeLists.txt

@@ -303,7 +303,7 @@ ENDIF()
 
 # Set commonly used variables
 IF(WIN32)
-  SET(DEFAULT_MYSQL_HOME "C:/MariaDB${MYSQL_BASE_VERSION}")
+  SET(DEFAULT_MYSQL_HOME "C:/Program Files/MariaDB ${MYSQL_BASE_VERSION}")
   SET(SHAREDIR share)
 ELSE()
   SET(DEFAULT_MYSQL_HOME ${CMAKE_INSTALL_PREFIX})