Avoid appending empty include dir to CPPFLAGS

Previously, this resulted in invalid compile options such as the following tokyocabinet test:
gcc -o conftest -pthread -g -O2 -O2 -DNDEBUG   -I/usr/local/include -I    conftest.c -ltokyocabinet
Merge pull request #890 from awsiv/fix_empty_include_dir

Avoid appending empty include dir to CPPFLAGS