plugins: kafka: fix cmake cross compile error

KAFKA_INCLUDEDIR is not set if FLB_PREFER_SYSTEM_LIB_KAFKA is not used,
when cross-compiling, it just translates to -I/librdkafka, which is not allowed.
Fix this by only including KAFKA_INCLUDEDIR if really set.

x86_64-linux-gcc: ERROR: unsafe header/library path used in cross-compilation: '-I/librdkafka'

Signed-off-by: Thomas Devoogdt <thomas@devoogdt.com>

Author: ThomasDevoogdt

plugins/in_kafka/CMakeLists.txt

@@ -3,5 +3,7 @@ set(src
   )
 
 FLB_PLUGIN(in_kafka "${src}" ${KAFKA_LIBRARIES} flb-aws)
-target_include_directories(flb-plugin-in_kafka PUBLIC ${KAFKA_INCLUDEDIR}/librdkafka)
+if(DEFINED KAFKA_INCLUDEDIR)
+  target_include_directories(flb-plugin-in_kafka PUBLIC ${KAFKA_INCLUDEDIR}/librdkafka)
+endif()
 target_link_libraries(flb-plugin-in_kafka -lpthread)

plugins/out_kafka/CMakeLists.txt

@@ -5,5 +5,7 @@ set(src
   kafka.c)
 
 FLB_PLUGIN(out_kafka "${src}" ${KAFKA_LIBRARIES} flb-aws)
-target_include_directories(flb-plugin-out_kafka PUBLIC ${KAFKA_INCLUDEDIR}/librdkafka)
+if(DEFINED KAFKA_INCLUDEDIR)
+  target_include_directories(flb-plugin-out_kafka PUBLIC ${KAFKA_INCLUDEDIR}/librdkafka)
+endif()
 target_link_libraries(flb-plugin-out_kafka -lpthread)