Move version number to separate file so CMake doesn't reconfigure when it changes

cmake/GenerateSoftFeverVersion.cmake

@@ -0,0 +1,25 @@
+if(NOT DEFINED INPUT)
+    message(FATAL_ERROR "INPUT is required")
+endif()
+
+if(NOT DEFINED OUTPUT)
+    message(FATAL_ERROR "OUTPUT is required")
+endif()
+
+file(STRINGS "${INPUT}" SOFTFEVER_VERSION LIMIT_COUNT 1)
+string(STRIP "${SOFTFEVER_VERSION}" SOFTFEVER_VERSION)
+
+set(CONTENT [=[
+#include "libslic3r_version.h"
+
+namespace Slic3r {
+
+const char* softfever_version()
+{
+    return "@SOFTFEVER_VERSION@";
+}
+
+} // namespace Slic3r
+]=])
+string(CONFIGURE "${CONTENT}" CONTENT @ONLY)
+file(WRITE "${OUTPUT}" "${CONTENT}")