title: C compiler identification is unknown despite setting CC and CXX variables
body: <p>I'm using the command:</p>
  
  <pre><code>    cmake CC="C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\bin\cl.exe" 
  CXX="C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\bin\cl.exe"  -G "Visual Studio 12
   Win64" -DBOOST_ROOT="c:\local\boost_1_56_0" -DBOOST_LIBRARYDIR="c:\local\boost_1_56_0\lib64-
  msvc-12.0"  ..
  </code></pre>
  
  <p>It says:</p>
  
  <pre><code>The C compiler identification is unknown
  </code></pre>
  
  <p>and </p>
  
  <pre><code>The CXX compiler identification is unknown
  </code></pre>
  
  <p>How is it possible when I set those variables right there?</p>
  
  <p><strong>EDIT</strong>: Error log file:</p>
  
  <pre><code>Compiling the C compiler identification source file "CMakeCCompilerId.c" failed.
  Compiler:  
  Build flags: 
  Id flags: 
  
  The output was:
  1
  Microsoft (R) Build Engine version 12.0.30723.0
  [Microsoft .NET Framework, version 4.0.30319.18444]
  Copyright (C) Microsoft Corporation. All rights reserved.
  
  Build started 21-09-2014 20:17:49.
  Project "D:\foo\build\CMakeFiles\3.0.2\CompilerIdC\CompilerIdC.vcxproj" on node 1 (default targets).
  PrepareForBuild:
    Creating directory "Debug\".
    Creating directory "Debug\CompilerIdC.tlog\".
  InitializeBuildStatus:
    Creating "Debug\CompilerIdC.tlog\unsuccessfulbuild" because "AlwaysCreate" was specified.
  ClCompile:
    C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\bin\x86_amd64\CL.exe /c /nologo /W0 /WX- /Od /D _MBCS /Gm- /EHsc /RTC1 /MDd /GS /fp:precise /Zc:wchar_t /Zc:forScope /Fo"Debug\\" /Fd"Debug\vc120.pdb" /Gd /TC /errorReport:queue CMakeCCompilerId.c
    CMakeCCompilerId.c
  Link:
    C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\bin\x86_amd64\link.exe /ERRORREPORT:QUEUE /OUT:".\CompilerIdC.exe" /INCREMENTAL:NO /NOLOGO kernel32.lib user32.lib gdi32.lib winspool.lib comdlg32.lib advapi32.lib shell32.lib ole32.lib oleaut32.lib uuid.lib odbc32.lib odbccp32.lib /MANIFEST /MANIFESTUAC:"level='asInvoker' uiAccess='false'" /manifest:embed /PDB:".\CompilerIdC.pdb" /SUBSYSTEM:CONSOLE /TLBID:1 /DYNAMICBASE /NXCOMPAT /IMPLIB:".\CompilerIdC.lib" /MACHINE:X64 Debug\CMakeCCompilerId.obj
  LINK : fatal error LNK1181: cannot open input file 'gdi32.lib' [D:\foo\build\CMakeFiles\3.0.2\CompilerIdC\CompilerIdC.vcxproj]
  Done Building Project "D:\foo\build\CMakeFiles\3.0.2\CompilerIdC\CompilerIdC.vcxproj" (default targets) -- FAILED.
  
  Build FAILED.
  
  "D:\foo\build\CMakeFiles\3.0.2\CompilerIdC\CompilerIdC.vcxproj" (default target) (1) -&gt;
  (Link target) -&gt; 
    LINK : fatal error LNK1181: cannot open input file 'gdi32.lib' [D:\foo\build\CMakeFiles\3.0.2\CompilerIdC\CompilerIdC.vcxproj]
  
      0 Warning(s)
      1 Error(s)
  
  Time Elapsed 00:00:00.47
  
  
  Compiling the CXX compiler identification source file "CMakeCXXCompilerId.cpp" failed.
  Compiler:  
  Build flags: 
  Id flags: 
  
  The output was:
  1
  Microsoft (R) Build Engine version 12.0.30723.0
  [Microsoft .NET Framework, version 4.0.30319.18444]
  Copyright (C) Microsoft Corporation. All rights reserved.
  
  Build started 21-09-2014 20:17:50.
  Project "D:\foo\build\CMakeFiles\3.0.2\CompilerIdCXX\CompilerIdCXX.vcxproj" on node 1 (default targets).
  PrepareForBuild:
    Creating directory "Debug\".
    Creating directory "Debug\CompilerIdCXX.tlog\".
  InitializeBuildStatus:
    Creating "Debug\CompilerIdCXX.tlog\unsuccessfulbuild" because "AlwaysCreate" was specified.
  ClCompile:
    C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\bin\x86_amd64\CL.exe /c /nologo /W0 /WX- /Od /D _MBCS /Gm- /EHsc /RTC1 /MDd /GS /fp:precise /Zc:wchar_t /Zc:forScope /Fo"Debug\\" /Fd"Debug\vc120.pdb" /Gd /TP /errorReport:queue CMakeCXXCompilerId.cpp
    CMakeCXXCompilerId.cpp
  Link:
    C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\bin\x86_amd64\link.exe /ERRORREPORT:QUEUE /OUT:".\CompilerIdCXX.exe" /INCREMENTAL:NO /NOLOGO kernel32.lib user32.lib gdi32.lib winspool.lib comdlg32.lib advapi32.lib shell32.lib ole32.lib oleaut32.lib uuid.lib odbc32.lib odbccp32.lib /MANIFEST /MANIFESTUAC:"level='asInvoker' uiAccess='false'" /manifest:embed /PDB:".\CompilerIdCXX.pdb" /SUBSYSTEM:CONSOLE /TLBID:1 /DYNAMICBASE /NXCOMPAT /IMPLIB:".\CompilerIdCXX.lib" /MACHINE:X64 Debug\CMakeCXXCompilerId.obj
  LINK : fatal error LNK1181: cannot open input file 'gdi32.lib' [D:\foo\build\CMakeFiles\3.0.2\CompilerIdCXX\CompilerIdCXX.vcxproj]
  Done Building Project "D:\foo\build\CMakeFiles\3.0.2\CompilerIdCXX\CompilerIdCXX.vcxproj" (default targets) -- FAILED.
  
  Build FAILED.
  
  "D:\foo\build\CMakeFiles\3.0.2\CompilerIdCXX\CompilerIdCXX.vcxproj" (default target) (1) -&gt;
  (Link target) -&gt; 
    LINK : fatal error LNK1181: cannot open input file 'gdi32.lib' [D:\foo\build\CMakeFiles\3.0.2\CompilerIdCXX\CompilerIdCXX.vcxproj]
  
      0 Warning(s)
      1 Error(s)
  
  Time Elapsed 00:00:00.50
  </code></pre>