Change CMake so that we only look for Z3 when LLVM_ENABLE_Z3_SOLVER is enabled

[mehdi_amini]

Seems like the previous behavior was to try to find a compatible Z3 and use it even if the user didn't opt-in.
I am not sure if there was a strong use case for this, I rather have something like this more explicitly configured.

So LGTM, but the original author of this configuration (or someone else knowledgeable about LLVM uses of Z3) should also chime in.

[mikhail.ramalho]

LGTM.

Do you have permission to push the patch? Otherwise, I can push it.

[hcorion]

AFAIK I don't have push permissions, thanks I appreciate that.

llvm/CMakeLists.txt

 option(LLVM_ENABLE_LIBPFM "Use libpfm for performance counters if available." ON)
 
 option(LLVM_ENABLE_THREADS "Use threads if available." ON)
 
 set(LLVM_ENABLE_ZLIB "ON" CACHE STRING "Use zlib for compression/decompression if available. Can be ON, OFF, or FORCE_ON")
 
 set(LLVM_Z3_INSTALL_DIR "" CACHE STRING "Install directory of the Z3 solver.")
 
+option(LLVM_ENABLE_Z3_SOLVER
+  "Enable Support for the Z3 constraint solver in LLVM."
+  ${LLVM_ENABLE_Z3_SOLVER_DEFAULT}
+)
+
+if (LLVM_ENABLE_Z3_SOLVER)
   find_package(Z3 4.7.1)
   
   if (LLVM_Z3_INSTALL_DIR)
     if (NOT Z3_FOUND)
       message(FATAL_ERROR "Z3 >= 4.7.1 has not been found in LLVM_Z3_INSTALL_DIR: ${LLVM_Z3_INSTALL_DIR}.")
     endif()
   endif()
   
-set(LLVM_ENABLE_Z3_SOLVER_DEFAULT "${Z3_FOUND}")
-
-option(LLVM_ENABLE_Z3_SOLVER
-  "Enable Support for the Z3 constraint solver in LLVM."
-  ${LLVM_ENABLE_Z3_SOLVER_DEFAULT}
-)
-
-if (LLVM_ENABLE_Z3_SOLVER)
   if (NOT Z3_FOUND)
     message(FATAL_ERROR "LLVM_ENABLE_Z3_SOLVER cannot be enabled when Z3 is not available.")
   endif()
 
   set(LLVM_WITH_Z3 1)
 endif()
 
+set(LLVM_ENABLE_Z3_SOLVER_DEFAULT "${Z3_FOUND}")
+
+
 if( LLVM_TARGETS_TO_BUILD STREQUAL "all" )
   set( LLVM_TARGETS_TO_BUILD ${LLVM_ALL_TARGETS} )
 endif()
 
 set(LLVM_TARGETS_TO_BUILD
    ${LLVM_TARGETS_TO_BUILD}
    ${LLVM_EXPERIMENTAL_TARGETS_TO_BUILD})
 list(REMOVE_DUPLICATES LLVM_TARGETS_TO_BUILD)