mongodb: Fix build with Xcode 10.2

Closes: https://trac.macports.org/ticket/58277

databases/mongodb/Portfile

@@ -79,6 +79,8 @@ if {[vercmp ${macosx_deployment_target} 10.11] >= 0 && [vercmp ${configure.sdk_v
     require_active_variants ld64 ld64_xcode
 }
 
+patchfiles          string_view.patch
+
 configure.cflags    {*}${configure.cc_archflags}
 configure.ldflags   {*}${configure.ld_archflags}
 

databases/mongodb/files/string_view.patch

@@ -0,0 +1,16 @@
+Minimal change to fix compilation with Xcode 10.2.
+https://jira.mongodb.org/browse/SERVER-37993
+https://github.com/boostorg/asio/commit/b5b17a67f0aa29f5156324d5e8a73dd8669a5a51
+--- src/third_party/asio-master/asio/include/asio/detail/config.hpp.orig	2018-07-03 21:09:33 UTC
++++ src/third_party/asio-master/asio/include/asio/detail/config.hpp
+@@ -784,8 +784,9 @@
+ # if !defined(ASIO_DISABLE_STD_STRING_VIEW)
+ #  if defined(__clang__)
+ #   if (__cplusplus >= 201402)
+-#    if __has_include(<experimental/string_view>)
++#    if __has_include(<string_view>)
+ #     define ASIO_HAS_STD_STRING_VIEW 1
++#    elif __has_include(<experimental/string_view>)
+ #     define ASIO_HAS_STD_EXPERIMENTAL_STRING_VIEW 1
+ #    endif // __has_include(<experimental/string_view>)
+ #   endif // (__cplusplus >= 201402)