[build] use <experimental/optional> polyfill

It is not available anymore in newer STL versions, e.g. according to libc++'s TS deprecation policy: https://libcxx.llvm.org/TS_deprecation.html

cmake/core.cmake

@@ -1,3 +1,5 @@
+add_vendor_target(optional INTERFACE)
+
 # Modify cmake/core-files.txt to change the source files for this target.
 load_sources_list(MBGL_CORE_FILES cmake/core-files.txt)
 add_library(mbgl-core STATIC ${MBGL_CORE_FILES})
@@ -7,7 +9,10 @@ target_include_directories(mbgl-core
     PRIVATE src
 )
 
-target_link_libraries(mbgl-core PRIVATE codecvt)
+target_link_libraries(mbgl-core
+    PUBLIC optional
+    PRIVATE codecvt
+)
 
 target_add_mason_package(mbgl-core PUBLIC geometry)
 target_add_mason_package(mbgl-core PUBLIC variant)

cmake/loop-darwin.cmake

@@ -7,6 +7,10 @@ target_include_directories(mbgl-loop-darwin
     PRIVATE src
 )
 
+target_link_libraries(mbgl-loop-darwin
+    PUBLIC optional
+)
+
 create_source_groups(mbgl-loop-darwin)
 
 set_target_properties(mbgl-loop-darwin PROPERTIES FOLDER "Core")

cmake/loop-uv.cmake

@@ -7,6 +7,7 @@ target_sources(mbgl-loop-uv INTERFACE
 )
 
 target_include_directories(mbgl-loop-uv INTERFACE
+    optional
     ${CMAKE_CURRENT_SOURCE_DIR}/include
     ${CMAKE_CURRENT_SOURCE_DIR}/src
 )

scripts/vendor/optional.sh

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+source "$(dirname "${BASH_SOURCE[0]}")/common.sh"
+
+NAME=optional
+VERSION=f6249e7fdcb80131c390a083f1621d96023e72e9
+ROOT=Optional-$VERSION
+
+download "https://github.com/akrzemi1/Optional/archive/$VERSION.tar.gz"
+init
+extract_gzip "$ROOT/optional.hpp" "$ROOT/copyright.txt" "$ROOT/LICENSE"
+mkdir -p include/experimental
+mv optional.hpp include/experimental/optional
+file_list include -type f

vendor/optional/files.txt

@@ -0,0 +1 @@
+include/experimental/optional