Make flex and bison binaries required.

Without them being required, a new build where
the binaries of the two programs are not found
fails late and with confusing error messages.

This makes the build fail fast and early, at
the point where the error is produced (where
the binaries have failed to have been found).

Author: NlightNFotis

src/CMakeLists.txt

@@ -1,7 +1,7 @@
 project(CBMC)
 
-find_package(BISON)
-find_package(FLEX)
+find_package(BISON REQUIRED)
+find_package(FLEX REQUIRED)
 
 find_package(Doxygen)
 if(DOXYGEN_FOUND)