omnitrace find_package support (#3)

* omnitrace find_package support

- Fix to INSTALL_DESTINATION for configure_package_config_file
- Fixes to ConfigInstall.cmake and omnitrace-config.cmake.in

* Test find_package

source/lib/omnitrace-user/user.cpp

@@ -20,7 +20,9 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
-#define OMNITRACE_SOURCE 1
+#if !defined(OMNITRACE_USER_SOURCE)
+#    define OMNITRACE_USER_SOURCE 1
+#endif
 
 #include "omnitrace/user.h"