Don't error out if the terminfo libraries aren't found

In current LLVM releases (10 and 11 at least), LLVM_ENABLE_TERMINFO
only indicates whether it tried to find a terminfo library, but the
LLVM cmake configuration might still have not found any library
despite that.

In the current LLVM master branch (upcoming 12 release),
LLVM_ENABLE_TERMINFO is updated after the fact, indicating whether
it actually ended up using (and requiring) such a library.

For cases where a terminfo library really is required but isn't
found, this pushes the errors from the cmake phase onto the linking
phase, but fixes cases with LLVM 10 and 11 (and possibly other older
releases as well) if LLVM didn't end up using any terminfo library.

Author: mstorsjo

src/CMakeLists.txt

@@ -97,7 +97,7 @@ if (NOT lib_llvm)
     list(APPEND llvm_deps "${lib}")
   endforeach()
   if (LLVM_ENABLE_TERMINFO AND NOT WIN32)
-    find_library(TERMINFO_LIB NAMES terminfo tinfo curses ncurses ncursesw REQUIRED)
+    find_library(TERMINFO_LIB NAMES terminfo tinfo curses ncurses ncursesw)
     list(APPEND llvm_deps "${TERMINFO_LIB}")
   endif()
 endif()