Allow build from uncommon bcc installation

I'm not able to build bpftrace with bcc being installed
under directory that is not searched by linker by dafault.

Adding -L<dirname ${LIBBCC_LIBDIR}> option, which allows me
to build with:
  $ cmake \
    ...
    -DLIBBCC_INCLUDE_DIRS:PATH=/opt/bcc/include/bcc \
    -DLIBBCC_LIBRARIES:PATH=/opt/bcc/lib64/libbcc.so \
    ...

CMakeLists.txt

@@ -64,6 +64,9 @@ if(POLICY CMP0075)
   cmake_policy(SET CMP0075 NEW)
 endif()
 set(CMAKE_REQUIRED_LIBRARIES bcc)
+get_filename_component(LIBBCC_LIBDIR ${LIBBCC_LIBRARIES} DIRECTORY)
+set(CMAKE_REQUIRED_LINK_OPTIONS -L${LIBBCC_LIBDIR})
+
 check_symbol_exists(bcc_prog_load "${LIBBCC_INCLUDE_DIRS}/libbpf.h" HAVE_BCC_PROG_LOAD)
 check_symbol_exists(bcc_create_map "${LIBBCC_INCLUDE_DIRS}/libbpf.h" HAVE_BCC_CREATE_MAP)
 check_symbol_exists(bcc_elf_foreach_sym "${LIBBCC_INCLUDE_DIRS}/bcc_elf.h" HAVE_BCC_ELF_FOREACH_SYM)