Fix public headers for make install. (#2856)

Fix public headers for make install. (#2856)

This commit fixes make install, which previously copied all public header files to ${CMAKE_INSTALL_PREFIX}/ instead of ${CMAKE_INSTALL_PREFIX}/cvc4.
Further, the old build system modified all #include directives in the installed public header files to use the installed headers, e.g., #include "cvc4_public.h" was changed to #include <cvc4/cvc4_public.h>. Now, after make install the script src/fix-install-headers.sh is executed to change the #include directives accordingly (this should be obsolete with the new C++ API).