Symbian: Fix pathes

Fix pathes for Nokia Qt SDK:
 * Set SBS path in one location only
 * Fix up perl path
 * Set path to CMD on windows. Both ABLD and SBSv2 actually need this.
 * Set GCC path only if the directory actually exists

These fixes allow for building both ABLD and SBSv2 based projects in
a clean environment.

Reviewed-by: Tim Jenssen