[3.15] gh-155911: Fix curses detection with libraries in LIBS (GH-156130)

AC_SEARCH_LIBS tries linking with no extra library first, so a curses
library already in LIBS made the search report "none required", which
then became the module's link flag.  Search with only the libraries the
module is linked with.
(cherry picked from commit f74cdf8)

Co-authored-by: Shamil <ashm.tech@proton.me>

Author: serhiy-storchaka

Misc/NEWS.d/next/Build/2026-08-20-17-41-11.gh-issue-155911.yL3z8q.rst

@@ -0,0 +1,2 @@
+Fix curses detection when a curses library is already in ``LIBS``.
+Patch by Shamil Abdulaev.

configure

@@ -7149,8 +7149,9 @@ WITH_SAVE_ENV([
   ]))
 
   # Check that we're able to link with crucial curses/panel functions. This
-  # also serves as a fallback in case pkg-config failed.
-  AS_VAR_APPEND([LIBS], [" $CURSES_LIBS $PANEL_LIBS"])
+  # also serves as a fallback in case pkg-config failed. Extension modules are
+  # not linked with LIBS, so exclude it to get the required libraries.
+  LIBS="$CURSES_LIBS $PANEL_LIBS"
   AC_SEARCH_LIBS([initscr], [ncursesw ncurses],
     [AS_VAR_IF([have_curses], [no],
       [AS_VAR_SET([have_curses], [yes])