build failure on Mac OS X with configure --enable-udp-l4s

~~~
g++ -std=gnu++11 -DHAVE_CONFIG_H -I. -I.. -I../include -I../include -Wall -O2 -g -O2 -MT prague_cc.o -MD -MP -MF .deps/prague_cc.Tpo -c -o prague_cc.o prague_cc.cpp
In file included from prague_cc.cpp:2:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/chrono:857:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/forward_list:1586:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/functional:526:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__functional/boyer_moore_searcher.h:27:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/vector:325:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__format/formatter_bool.h:20:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__format/formatter_integral.h:35:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/locale:202:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__locale:20:
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/clocale:43:5: error: <clocale> tried including <locale.h> but didn't find libc++'s <locale.h> header. This usually means that your header search paths are not configured properly. The header search paths should contain the C++ Standard Library headers before any C Standard Library, and you are probably using compiler flags that make that not be the case.
43 | # error <clocale> tried including <locale.h> but didn't find libc++'s <locale.h> header. \
| ^
1 error generated.
make[2]: *** [prague_cc.o] Error 1
make[1]: *** [all-recursive] Error 1
make: *** [all] Error 2~~~</locale.h></locale.h></clocale></locale.h></locale.h></clocale>