gh-115119: Fall back to bundled libmpdec if system libmpdec is not found (GH-119196)

(cherry picked from commit 642b25b)

Co-authored-by: Erlend E. Aasland <erlend@python.org>

Author: erlend-aasland

Misc/NEWS.d/next/Core and Builtins/2024-05-19-22-54-55.gh-issue-115119.DwMwev.rst

@@ -0,0 +1 @@
+Fall back to the bundled libmpdec if a system version cannot be found.

configure

@@ -3980,16 +3980,21 @@ AC_ARG_WITH(
   [with_system_libmpdec="yes"])
 AC_MSG_RESULT([$with_system_libmpdec])
 
+AC_DEFUN([USE_BUNDLED_LIBMPDEC],
+         [LIBMPDEC_CFLAGS="-I\$(srcdir)/Modules/_decimal/libmpdec"
+          LIBMPDEC_LIBS="-lm \$(LIBMPDEC_A)"
+          LIBMPDEC_INTERNAL="\$(LIBMPDEC_HEADERS) \$(LIBMPDEC_A)"
+          have_mpdec=yes
+          with_system_libmpdec=no])
+
 AS_VAR_IF(
   [with_system_libmpdec], [yes],
   [PKG_CHECK_MODULES(
     [LIBMPDEC], [libmpdec >= 2.5.0], [],
     [LIBMPDEC_CFLAGS=${LIBMPDEC_CFLAGS-""}
      LIBMPDEC_LIBS=${LIBMPDEC_LIBS-"-lmpdec -lm"}
      LIBMPDEC_INTERNAL=])],
-  [LIBMPDEC_CFLAGS="-I\$(srcdir)/Modules/_decimal/libmpdec"
-   LIBMPDEC_LIBS="-lm \$(LIBMPDEC_A)"
-   LIBMPDEC_INTERNAL="\$(LIBMPDEC_HEADERS) \$(LIBMPDEC_A)"])
+  [USE_BUNDLED_LIBMPDEC()])
 
 AS_VAR_IF([with_system_libmpdec], [yes],
   [WITH_SAVE_ENV([
@@ -4006,13 +4011,15 @@ AS_VAR_IF([with_system_libmpdec], [yes],
       [have_mpdec=yes],
       [have_mpdec=no])
   ])],
-  [AS_VAR_SET([have_mpdec], [yes])
-   AC_MSG_WARN([m4_normalize([
+  [AC_MSG_WARN([m4_normalize([
      the bundled copy of libmpdecimal is scheduled for removal in Python 3.15;
      consider using a system installed mpdecimal library.])])])
 
 AS_IF([test "$with_system_libmpdec" = "yes" && test "$have_mpdec" = "no"],
-      [AC_MSG_WARN([no system libmpdecimal found; unable to build _decimal])])
+      [AC_MSG_WARN([m4_normalize([
+         no system libmpdecimal found; falling back to bundled libmpdecimal
+         (deprecated and scheduled for removal in Python 3.15)])])
+       USE_BUNDLED_LIBMPDEC()])
 
 # Disable forced inlining in debug builds, see GH-94847
 AS_VAR_IF(