Fix edge case build failure triggered by passing every
DISABLE_FIND_PACKAGE, including disabling the normally
always-enabled readline.

CMake Error at CMakeLists.txt:229 (check_symbol_exists):
  Unknown CMake command "check_symbol_exists".
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -12,2 +12,3 @@
 include(CheckStructHasMember)
+include(CheckSymbolExists)
 include(CheckCCompilerFlag)