Conditionally replace semicolon with escaped char to fix VS2010/VS2012 errors

The build error on VS2010 was:

 Error 30 error MSB6001: Invalid command line switch for "CL.exe". The
 value [PYTHONPATH="] contains an odd number of double-quote characters.
 Only even numbers of literal double-quote characters are acceptable to
 command line tools. C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\Platforms\Win32\Microsoft.Cpp.Win32.Targets 147

The error is also reported on the Microsoft Article ID: 2262855 and
the suggested solution is to change the semicolon into an XML escaped
string '%3B'.
More details here: http://support.microsoft.com/kb/2262855

The PreprocessorDefinitions element within libpython-shared.vcxproj file
generated for VS2010 was:

(1) *With* patch:
    <PreprocessorDefinitions Condition="'$(Configuration)|$(Platform)'=='Release|Win32'">LANDMARK="Lib\\os.py";PYTHONPATH="%3B.\\Lib%3B.\\Lib\\lib-dynload%3B.\\Lib\\lib-dynload\\$(Configuration)%3B.\\Lib\\plat-win32";%(PreprocessorDefinitions)</PreprocessorDefinitions>

(2) *Without* patch:
    <PreprocessorDefinitions Condition="'$(Configuration)|$(Platform)'=='Release|Win32'">LANDMARK="Lib\\os.py";PYTHONPATH=";.\\Lib;.\\Lib\\lib-dynload;.\\Lib\\lib-dynload\\$(Configuration);.\\Lib\\plat-win32";%(PreprocessorDefinitions)</PreprocessorDefinitions>

The first version of this patch was changing the semicolon into '%3B'
independently of the Visual Studio version. While the project was
compiling on both VS2008 and VS2010, the python interpreter built
using VS2008 couldn't be started from either a build or install tree.
This was explained by the fact the "%" was escaped with an other "%"
character: http://cmake.org/gitweb?p=cmake.git;a=blob;f=Source/kwsys/System.c;h=5d178bfa27b98f562fd4e126849a0db5b451af63;hb=HEAD#l506

The PreprocessorDefinitions element within libpython-shared.vcproj file
generated for VS2008 was:

(1) *Without* '%3B':
   PreprocessorDefinitions="LANDMARK=\&quot;Lib\\os.py\&quot;;PYTHONPATH=\&quot;;.\\Lib;.\\Lib\\lib-dynload;.\\Lib\\lib-dynload\\$(OutDir);.\\Lib\\plat-win32\&quot;"

(2) *With* '%3B':
   PreprocessorDefinitions="LANDMARK=\&quot;Lib\\os.py\&quot;;PYTHONPATH=\&quot;%%3B.\\Lib%%3B.\\Lib\\lib-dynload%%3B.\\Lib\\lib-dynload\\$(OutDir)%%3B.\\Lib\\plat-win32\&quot;"

Due to an incorrect escaping the PYTHONPATH was containing path separated
with string "%3B" whereas ";" was expected. This is very probably a CMake
bug. See http://cmake.org/Bug/view.php?id=14073
Merge branch 'fix-msvc2012-build-error'

* fix-msvc2012-build-error:
  Conditionally replace semicolon with escaped char to fix VS2010/VS2012 errors