fix: no gmp.h should not result in configure error for `--with-back…

…end=auto`

configure.ac

@@ -159,37 +159,39 @@ if test x"$has_gmp" != x"yes"; then
   AC_CHECK_HEADER(
     gmp.h,
     [AC_CHECK_LIB(gmp, __gmpz_init, [], [AC_MSG_ERROR([libgmp not found or unusable])])],
-    [AC_MSG_ERROR([gmp header not found])]
+    [has_gmp=no]
   )
 
-  # Proper support for macOS aarch64 was introduced in 6.3.0, but LTS distros
-  # like Ubuntu 20.04 (focal) use 6.2.0, so, the minimum supported version is
-  # determined by platform and architecture.
-  case $host in
-    *darwin*)
-      case $host_cpu in
-        aarch*)
-          gmp_major_version=6
-          gmp_minor_version=3
-          ;;
-      esac
-      ;;
-    *)
-      gmp_major_version=6
-      gmp_minor_version=2
-      ;;
-  esac
-  AC_MSG_CHECKING([gmp version >= $gmp_major_version.$gmp_minor_version])
-  AC_COMPILE_IFELSE(
-    [AC_LANG_PROGRAM(
-      [[@%:@include <gmp.h>]],
-      [[
-        @%:@if __GNU_MP_VERSION < $gmp_major_version || __GNU_MP_VERSION_MINOR < $gmp_minor_version
-          @%:@error Unsupported GMP version
-        @%:@endif]])],
-    [AC_MSG_RESULT([yes]); has_gmp=yes; GMP_LIBS="-lgmp";],
-    [AC_MSG_RESULT([no]); AC_MSG_ERROR([unsupported gmp version])]
-  )
+  if test x"$has_gmp" != x"no"; then
+    # Proper support for macOS aarch64 was introduced in 6.3.0, but LTS distros
+    # like Ubuntu 20.04 (focal) use 6.2.0, so, the minimum supported version is
+    # determined by platform and architecture.
+    case $host in
+      *darwin*)
+        case $host_cpu in
+          aarch*)
+            gmp_major_version=6
+            gmp_minor_version=3
+            ;;
+        esac
+        ;;
+      *)
+        gmp_major_version=6
+        gmp_minor_version=2
+        ;;
+    esac
+    AC_MSG_CHECKING([gmp version >= $gmp_major_version.$gmp_minor_version])
+    AC_COMPILE_IFELSE(
+      [AC_LANG_PROGRAM(
+        [[@%:@include <gmp.h>]],
+        [[
+          @%:@if __GNU_MP_VERSION < $gmp_major_version || __GNU_MP_VERSION_MINOR < $gmp_minor_version
+            @%:@error Unsupported GMP version
+          @%:@endif]])],
+      [AC_MSG_RESULT([yes]); has_gmp=yes; GMP_LIBS="-lgmp";],
+      [AC_MSG_RESULT([no]); AC_MSG_ERROR([unsupported gmp version])]
+    )
+  fi
 
   CPPFLAGS="$CPPFLAGS_TEMP"
   LIBS="$LIBS_TEMP"