pcre2test: avoid using readline headers with libedit

When asked to enable libedit in a system that ALSO has readline,
the headers of the former would be found and used by the earlier.

While that would mostly work, some functions will be missing
definitions (which is forbidden in C99), so instead abort the
configuration and let the user provide for them.

cmake/FindEditline.cmake

@@ -6,7 +6,6 @@ else(EDITLINE_INCLUDE_DIR AND EDITLINE_LIBRARY AND NCURSES_LIBRARY)
   FIND_PATH(EDITLINE_INCLUDE_DIR readline.h PATH_SUFFIXES
     editline
     edit/readline
-    readline
   )
 
   FIND_LIBRARY(EDITLINE_LIBRARY NAMES edit)

configure.ac

@@ -601,7 +601,7 @@ fi
 # headers in different places. Try to cover the most common ones.
 
 if test "$enable_pcre2test_libedit" = "yes"; then
-  AC_CHECK_HEADERS([editline/readline.h edit/readline/readline.h readline/readline.h], [
+  AC_CHECK_HEADERS([editline/readline.h edit/readline/readline.h], [
     HAVE_LIBEDIT_HEADER=1
     break
   ])
@@ -942,7 +942,7 @@ if test "$enable_pcre2test_libedit" = "yes"; then
   fi
   if test -z "$HAVE_LIBEDIT_HEADER"; then
     echo "** Cannot --enable-pcre2test-libedit because neither editline/readline.h"
-    echo "** nor readline/readline.h was found."
+    echo "** nor edit/readline/readline.h was found."
     exit 1
   fi
   if test -z "$LIBEDIT"; then