gh-115119: Detect _decimal dependencies using pkg-config (#115406)

pkg-config is supported for libmpdec 4.0.0 and newer.

Doc/using/configure.rst

@@ -389,6 +389,17 @@ Options for third-party dependencies
    C compiler and linker flags for ``libffi``, used by :mod:`ctypes` module,
    overriding ``pkg-config``.
 
+.. option:: LIBMPDEC_CFLAGS
+.. option:: LIBMPDEC_LIBS
+
+   C compiler and linker flags for ``libmpdec``, used by :mod:`decimal` module,
+   overriding ``pkg-config``.
+
+   .. note::
+
+      These environment variables have no effect unless
+      :option:`--with-system-libmpdec` is specified.
+
 .. option:: LIBLZMA_CFLAGS
 .. option:: LIBLZMA_LIBS
 
@@ -798,6 +809,8 @@ Libraries options
 
    .. versionadded:: 3.3
 
+   .. seealso:: :option:`LIBMPDEC_CFLAGS` and :option:`LIBMPDEC_LIBS`.
+
 .. option:: --with-readline=readline|editline
 
    Designate a backend library for the :mod:`readline` module.

Misc/NEWS.d/next/Build/2024-02-13-15-31-28.gh-issue-115119.FnQzAW.rst

@@ -0,0 +1,2 @@
+:program:`configure` now uses :program:`pkg-config` to detect :mod:`decimal`
+dependencies if the :option:`--with-system-libmpdec` option is given.

Modules/_decimal/_decimal.c

@@ -34,7 +34,19 @@
 #include "pycore_pystate.h"       // _PyThreadState_GET()
 #include "pycore_typeobject.h"
 #include "complexobject.h"
-#include "mpdecimal.h"
+
+#include <mpdecimal.h>
+
+// Reuse config from mpdecimal.h if present.
+#if defined(MPD_CONFIG_64)
+  #ifndef CONFIG_64
+    #define CONFIG_64 MPD_CONFIG_64
+  #endif
+#elif defined(MPD_CONFIG_32)
+  #ifndef CONFIG_32
+    #define CONFIG_32 MPD_CONFIG_32
+  #endif
+#endif
 
 #include <ctype.h>                // isascii()
 #include <stdlib.h>