cmake: z3: Remove EXACT from 4.7.1 after being compatible with 4.8.1

After check-in of D54391 a comment there by @mikhail.ramalho says:
Since we're supporting version 4.8.1 now, the cmake file should be changed to
"minimum" instead of "exact".

Differential Revision: https://reviews.llvm.org/D54535

llvm-svn: 347152