Update build.ps1 to set CEF_RUNTIME_LIBRARY_FLAG (MDd/MD)

[amaitland]

CEF added support for specifying MD as part of the cmake generation process. See https://bitbucket.org/chromiumembedded/cef/pull-requests/149/add-new-option-cef_runtime_library_flag for details.

We can remove the hack of manually editing the files now.

[amaitland]

Change made in 75edf68

Not tested yet.

[amaitland]

Changes reverted

[amaitland]

Second attempt in 72b7113

According to https://bitbucket.org/chromiumembedded/cef/wiki/LinkingDifferentRunTimeLibraries.md the syntax was incorrect. Updated with the example given.

[amaitland]

Build successful https://ci.appveyor.com/project/cefsharp/cef-binary/build/69.0.0-CI71