generate both version.h and version from CMakelists.txt

This forces consistency, since they will be re-generated whenever
a git operation alters CMakelists.txt. They are still in the repo
because users might not actually run cmake.
diff --git a/version.in b/version.in
new file mode 100644
index 0000000..bfc03f7
--- /dev/null
+++ b/version.in
@@ -0,0 +1 @@
+@JSONCPP_VERSION@