Don't use precompiled headers for command-line based VC builds.
The reason, why libgit2 currently cannot support compiling via the command-line
tools cl/nmake from WinSDK and/or Microsoft Visual Studio, seems to be a
missing dependency on the generated precompiled header file.
The Visual Studio IDE automatically inserts this dependency when it sees the
right combination of "/Y" parameters.
This patch allows to compile using command line tools by disabling precompiled
headers for NON-IDE builds.