Improve error messages if configure build test fails under macOS

Telling people to try using a different SDK was rather confusing if they
didn't specify any SDK in the first place and not very helpful if the
problem was due to specifying an incompatible minimum OS version, so fix
both problems at once by using the appropriate error message depending
on the configure options actually used.

configure.in

@@ -1253,6 +1253,8 @@ if test "x$wxUSE_MACOSX_SDK" = "xno"; then
 elif test "x$wxUSE_MACOSX_SDK" = "xyes"; then
     dnl Try to use the default SDK.
     wxUSE_MACOSX_SDK="`xcode-select -p`/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk"
+elif test "x$wxUSE_MACOSX_SDK" != "x"; then
+    macosx_sdk_specified=yes
 fi
 
 
@@ -1305,6 +1307,8 @@ dnl We need to quote the next line where we don't need macros and do need [] in
     fi
 elif test "x$wxUSE_MACOSX_VERSION_MIN" = "x"; then
     wxUSE_MACOSX_VERSION_MIN=10.10
+else
+    macosx_minver_specified=yes
 fi
 
 if test "x$MACOSX_SDK_OPTS" != "x"; then
@@ -1341,14 +1345,22 @@ dnl If either an SDK or a version option was added, make sure that we can
 dnl still compile and link both C and C++.  If we didn't do this, then most
 dnl of the remaining tests would fail.
 if test "x$retest_macosx_linking" = "xyes"; then
+    if test "x$macosx_sdk_specified" = "xyes"; then
+        error_message="no, try using a different SDK or using the default one."
+    elif test "x$macosx_minver_specified" = "xyes"; then
+        error_message="no, try using a different min OS version or omitting it."
+    else
+        error_message="no, check that Xcode command line tools are installed."
+    fi
+
     AC_LANG_PUSH(C)
     AC_MSG_CHECKING([if C compiler ($CC) works with SDK/version options])
-    AC_TRY_LINK([],[],[AC_MSG_RESULT([yes])],[AC_MSG_FAILURE([no.  Try a different SDK]); exit 1])
+    AC_TRY_LINK([],[],[AC_MSG_RESULT([yes])],[AC_MSG_ERROR([$error_message])])
     AC_LANG_POP()
 
     AC_LANG_PUSH(C++)
     AC_MSG_CHECKING([if C++ compiler ($CXX) works with SDK/version options])
-    AC_TRY_LINK([],[],[AC_MSG_RESULT([yes])],[AC_MSG_FAILURE([no.  Try a different SDK]); exit 1])
+    AC_TRY_LINK([],[],[AC_MSG_RESULT([yes])],[AC_MSG_ERROR([$error_message])])
     AC_LANG_POP()
 fi