New 'configure' arg --with-mini-gmp

* configure.ac: It lets the builder override default of whther
mini-gmp is used.  Use AC_SEARCH_LIBS as per Autoconf manual.

configure.ac

@@ -4302,19 +4302,32 @@ AC_SUBST(KRB5LIB)
 AC_SUBST(DESLIB)
 AC_SUBST(KRB4LIB)
 
+AC_ARG_WITH([mini-gmp],
+  [AS_HELP_STRING([--without-mini-gmp],
+		  [don't compile and use mini-gmp, a substitute for the
+		   GNU Multiple Precision (GMP) library; this is the
+		   default on systems with recent-enough GMP.])])
 GMP_LIB=
-GMP_OBJ=
+GMP_OBJ=mini-gmp.o
 HAVE_GMP=no
-AC_CHECK_LIB(gmp, __gmpz_roinit_n, [
-  AC_CHECK_HEADERS(gmp.h, [
-    GMP_LIB=-lgmp
-    HAVE_GMP=yes
-    AC_DEFINE(HAVE_GMP, 1, [Define to 1 if you have gmp.h and -lgmp])])])
-if test $HAVE_GMP = no; then
-   GMP_OBJ=mini-gmp.o
+case $with_mini_gmp in
+  yes) ;;
+  no) HAVE_GMP=yes;;
+  *) AC_CHECK_HEADERS([gmp.h],
+       [OLIBS=$LIBS
+	AC_SEARCH_LIBS([__gmpz_roinit_n], [gmp])
+	LIBS=$OLIBS
+	case $ac_cv_search___gmpz_roinit_n in
+	  'none needed') HAVE_GMP=yes;;
+	  -*) HAVE_GMP=yes GMP_LIB=$ac_cv_search___gmpz_roinit_n;;
+	esac]);;
+esac
+if test "$HAVE_GMP" = yes; then
+  GMP_OBJ=
+  AC_DEFINE([HAVE_GMP], 1, [Define to 1 if you have recent-enough GMP.])
 fi
-AC_SUBST(GMP_LIB)
-AC_SUBST(GMP_OBJ)
+AC_SUBST([GMP_LIB])
+AC_SUBST([GMP_OBJ])
 
 AC_CHECK_HEADERS(valgrind/valgrind.h)
 

etc/NEWS

@@ -24,8 +24,11 @@ When you add a new item, use the appropriate mark if you are sure it applies,
 
 * Installation Changes in Emacs 27.1
 
-** configure now checks for the GMP library.  If not found, the
-included "mini-gmp" library is used instead.
+** Emacs now uses GMP, the GNU Multiple Precision library.
+By default, if 'configure' does not find a suitable libgmp, it
+arranges for the included mini-gmp library to be built and used.
+The new 'configure' option --with-mini-gmp uses mini-gmp even if a
+suitable libgmp is available.
 
 ** The new configure option '--with-json' adds support for JSON using
 the Jansson library.  It is on by default; use 'configure