Update documentation wrt build of VSC_main.h

https://github.com/varnishcache/varnish-cache/commit/dcaf616c66d93de69735237967cc091fa490bb93
removed the option to specifically build a single VSC header.

Closes #56

INSTALL.rst

@@ -109,7 +109,7 @@ Build required components from varnish-cache::
     pushd include && \
     make vcs_version.h && \
     popd && pushd lib/libvsc &&
-    make VSC_main.c \
+    make \
     )
 
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