Fix for users with no C++ support in their toolchain.

The U_CFG_GEOFENCE feature introduced a C++ file (common/geofence/src/u_geofence_geodesic.cpp) into the system, which does nothing if U_CFG_GEOFENCE is not defined, and even then only does something useful if geodesic is included in UBXLIB_FEATURES.  This causes a problem for customers who don't have C++ support in their toolchain; those who aren't interested in U_CFG_GEOFENCE or those who are but don't need the geodesic support (which is where C++ comes in).

With this commit the existing dummy file, common/geofence/src/dummy/u_geofence_geodesic.c (used for those who only have make, not CMake) is brought in if the geodesic feature is not included; this fixes things for both cases.

common/geofence/src/dummy/README.md

@@ -1,2 +1,2 @@
 # Why This File Is Here
-This file is here solely to allow those who only have a `make` (rather than `CMake`) based platform, and so are unable to use the third-party GeographicLib integration, to still compile and use the `ubxlib` GNSS geofence feature with a simple spherical earth model.  See the comments at the top of [u_geofence_geodesic.h](/common/geofence/api/u_geofence_geodesic.h) for more information. 
+This file is here solely to allow those who only have a `make` (rather than `CMake`) based platform, or do not have a C++ compiler, and so are unable to use the third-party GeographicLib integration, to still compile and use the `ubxlib` GNSS geofence feature with a simple spherical earth model.  See the comments at the top of [u_geofence_geodesic.h](/common/geofence/api/u_geofence_geodesic.h) for more information. 

port/ubxlib.cmake

@@ -140,7 +140,15 @@ list(APPEND UBXLIB_INC ${UBXLIB_BASE}/common/device/api)
 list(APPEND UBXLIB_PRIVATE_INC ${UBXLIB_BASE}/common/device/src)
 
 # CPP file required for geofencing
-list(APPEND UBXLIB_SRC ${UBXLIB_BASE}/common/geofence/src/u_geofence_geodesic.cpp)
+# Note: bringing the .c version of this file in if geodesic is not present is not
+# strictly necessary (since the file includes dummy implementations anyway); it
+# is done in order to allow those who don't have C++ support in their toolchain
+# to still compile/use the geofence feature in non-geodesic mode.
+if (geodesic IN_LIST UBXLIB_FEATURES)
+  list(APPEND UBXLIB_SRC ${UBXLIB_BASE}/common/geofence/src/u_geofence_geodesic.cpp)
+else()
+  list(APPEND UBXLIB_SRC ${UBXLIB_BASE}/common/geofence/src/dummy/u_geofence_geodesic.c)
+endif()
 
 # Default malloc()/free() implementation
 list(APPEND UBXLIB_SRC ${UBXLIB_BASE}/port/u_port_heap.c)
@@ -227,7 +235,7 @@ u_add_test_source_dir(base ${UBXLIB_BASE}/example/utilities/c030_module_fw_updat
 # cause any extra libraries to be linked and
 # UBXLIB_COMPILE_OPTIONS, which can be added to
 # target_compile_definitions(). HOWEVER, it doesn't
-# work for ESP-IDF, which has a "helful" component
+# work for ESP-IDF, which has a "helpful" component
 # system of its own stuck on top, hence BE AWARE
 # THAT that ESP-IDF doesn't use the bit below...
 #