/usr/lib/pkgconfig/libccgnu2.pc contains ccincludedir=${includedir}/cc++2 Cflags: -D_GNU_SOURCE -I${includedir}/cc++2 despite the fact that the headers are in /usr/include/cc++/ and /usr/include/cc++2/ doesn't exist. This causes problems compiling against them, because the headers cannot be found by the compiler if the make file uses pkg-config.
Upstream fixed that in a newer version, version bump to 1.5.7 should therefore fix this. Thanks for reporting!