address issue in boost.asio building on iOS

.github/workflows/macos.yml

@@ -120,7 +120,18 @@ jobs:
 
       - name: install boost
         run: |
-          brew install boost-build boost
+          git clone --depth=1 --recurse-submodules -j10 --branch=boost-1.78.0 https://github.com/boostorg/boost.git
+          cd boost
+          ./bootstrap.sh
+
+      - name: boost headers
+        run: |
+          cd boost
+          ./b2 headers
+          cp b2 ..
+
+      - name: user-config
+        run: |
           echo "using darwin : ios_sim : clang++ : <compileflags>-Wno-deprecated-declarations
           <compileflags>\"-isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk\"
           <compileflags>-mios-simulator-version-min=7
@@ -141,4 +152,4 @@ jobs:
 
       - name: build library
         run: |
-          b2 -l400 cxxstd=14 darwin-ios darwin-ios_sim address-model=64  link=static
+          BOOST_ROOT=boost ./b2 -l400 cxxstd=14 target-os=iphone crypto=built-in darwin-ios darwin-ios_sim address-model=64  link=static

Jamfile

@@ -155,6 +155,14 @@ rule linking ( properties * )
 		result += <framework>CoreFoundation <framework>SystemConfiguration ;
 	}
 
+	if <target-os>iphone in $(properties)
+	{
+		# boost.asio seems to mis-detect iOS as supporting the __thread
+		# keyword, resulting in the error:
+		# error: thread-local storage is not supported for the current target
+		result += <define>BOOST_ASIO_DISABLE_THREAD_KEYWORD_EXTENSION ;
+	}
+
 	if <toolset>gcc in $(properties)
 		&& <target-os>linux in $(properties)
 		&& ( <asserts>on in $(properties)