Fix is_root check to account for Windows paths.

Windows drive style paths should also be considered rooted as otherwise
we can't bootstrap when we specify one of those paths. This caused a
regression in Boost where that is regularly used.

azure-pipelines.yml

@@ -578,6 +578,10 @@ stages:
       vmImage: 'windows-latest'
     strategy:
       matrix:
+        1.73.0 .. VS 2019:
+          BOOST_VERSION: 1.73.0
+          BOOST_VERSION_U: 1_73_0
+          TOOLSET: vc142
         1.72.0 .. VS 2019:
           BOOST_VERSION: 1.72.0
           BOOST_VERSION_U: 1_72_0

src/engine/pathsys.h

@@ -99,7 +99,14 @@ namespace b2
     {
         inline bool is_rooted(const std::string &p)
         {
-            return p[0] == '/' || p[0] == '\\';
+            #if NT
+            return
+                (p.size() >= 1 && (p[0] == '/' || p[0] == '\\')) ||
+                (p.size() >= 3 && p[1] == ':' && (p[2] == '/' || p[2] == '\\'));
+            #else
+            return
+                (p.size() >= 1 && (p[0] == '/' || p[0] == '\\'));
+            #endif
         }
         std::string normalize(const std::string &p);
     }