CMake: Fix search for static libraries (#2798)

CMake: Fix search for static libraries (#2798)

When configuring CVC4 with `--static`, we change
`CMAKE_FIND_LIBRARY_SUFFIXES` to prefer static libraries (`*.a`) over
shared ones. However, instead of prepending `.a` to the list of
`CMAKE_FIND_LIBRARY_SUFFIXES`, we created a single element with `.a` and
the previous list.

Output of `message("${CMAKE_FIND_LIBRARY_SUFFIXES}")` before the change:

```
.a .tbd;.dylib;.so;.a
```

After the change:

```
.a;.tbd;.dylib;.so;.a
```

On macOS, both the static and the shared library of GMP are available
(when installed via homebrew) and before the change, CMake would pick the
shared library when compiling with `--static --no-static-binary`. This
commit fixes that issue.