Issue 2975: when compiling multiple extension modules with visual studio 2008

from the same python instance, some environment variables (LIB, INCLUDE)
would grow without limit.

Tested with these statements:
    distutils.ccompiler.new_compiler().initialize()
    print os.environ['LIB']
But I don't know how to turn them into reliable unit tests.

msvc9compiler.py

@@ -193,6 +193,17 @@ def normalize_and_reduce_paths(paths):
             reduced_paths.append(np)
     return reduced_paths
 
+def removeDuplicates(variable):
+    """Remove duplicate values of an environment variable.
+    """
+    oldList = variable.split(os.pathsep)
+    newList = []
+    for i in oldList:
+        if i not in newList:
+            newList.append(i)
+    newVariable = os.pathsep.join(newList)
+    return newVariable
+
 def find_vcvarsall(version):
     """Find the vcvarsall.bat file
 
@@ -252,12 +263,12 @@ def query_vcvarsall(version, arch="x86"):
         if '=' not in line:
             continue
         line = line.strip()
-        key, value = line.split('=')
+        key, value = line.split('=', 1)
         key = key.lower()
         if key in interesting:
             if value.endswith(os.pathsep):
                 value = value[:-1]
-            result[key] = value
+            result[key] = removeDuplicates(value)
 
     if len(result) != len(interesting):
         raise ValueError(str(list(result.keys())))