Only create uninstall target if it does not exist

In case librtlsdr is included as a subdirectory into another cmake
project the uninstall target may already be set and causes cmake
to fail. In a standalone build the check this patch adds does not
alter the behaviour.

(cherry picked from commit 0990db4247f16878d4f2765dd7d545080574ca79)

CMakeLists.txt

@@ -148,14 +148,17 @@ include_directories(
 ########################################################################
 # Create uninstall target
 ########################################################################
-configure_file(
-    ${PROJECT_SOURCE_DIR}/cmake/cmake_uninstall.cmake.in
-    ${CMAKE_CURRENT_BINARY_DIR}/cmake_uninstall.cmake
-@ONLY)
-
-add_custom_target(uninstall
-    ${CMAKE_COMMAND} -P ${CMAKE_CURRENT_BINARY_DIR}/cmake_uninstall.cmake
-)
+if(NOT TARGET uninstall)
+    configure_file(
+        ${PROJECT_SOURCE_DIR}/cmake/cmake_uninstall.cmake.in
+        ${CMAKE_CURRENT_BINARY_DIR}/cmake_uninstall.cmake
+        @ONLY
+    )
+
+    add_custom_target(uninstall
+        ${CMAKE_COMMAND} -P ${CMAKE_CURRENT_BINARY_DIR}/cmake_uninstall.cmake
+    )
+endif()
 
 ########################################################################
 # Install udev rules