The AX_CHECK_JEMALLOC() m4 macro sets the JEMALLOC_CFLAGS variable, not
JEMALLOC_CPPFLAGS. Furthermore, the JEMALLOC_CFLAGS and JEMALLOC_LIBS
variables should only be included in the build flags if jemalloc was
successfully configured. Tweak lib/isc/Makefile.am accordingly.