Merge pull request #2086 from randomascii/compiler_detect

Detect whether VC++ compiler can be found in configure.py

configure.py

@@ -305,9 +305,18 @@ def binary(name):
 else:
     n.variable('ar', configure_env.get('AR', 'ar'))
 
+def search_system_path(file_name):
+  """Find a file in the system path."""
+  for dir in os.environ['path'].split(';'):
+    path = os.path.join(dir, file_name)
+    if os.path.exists(path):
+      return path
+
 # Note that build settings are separately specified in CMakeLists.txt and
 # these lists should be kept in sync.
 if platform.is_msvc():
+    if not search_system_path('cl.exe'):
+        raise Exception('cl.exe not found. Run again from the Developer Command Prompt for VS')
     cflags = ['/showIncludes',
               '/nologo',  # Don't print startup banner.
               '/Zi',  # Create pdb with debug info.