Introduce BCC_KERNEL_MODULES_DIR

Add support for systems using a non-default ("/lib/modules") kernel lib
directory.

CMakeLists.txt

@@ -39,6 +39,12 @@ FOREACH(DIR ${LLVM_INCLUDE_DIRS})
   include_directories("${DIR}/../tools/clang/include")
 ENDFOREACH()
 
+# Set to a string path if system places kernel lib directory in
+# non-default location.
+if(NOT DEFINED BCC_KERNEL_MODULES_DIR)
+  set(BCC_KERNEL_MODULES_DIR "/lib/modules")
+endif()
+
 # Set to non-zero if system installs kernel headers with split source and build
 # directories in /lib/modules/`uname -r`/. This is the case for debian and
 # suse, to the best of my knowledge.

src/cc/frontends/clang/CMakeLists.txt

 # Copyright (c) PLUMgrid, Inc.
 # Licensed under the Apache License, Version 2.0 (the "License")
 
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DKERNEL_MODULES_DIR='\"${BCC_KERNEL_MODULES_DIR}\"'")
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DKERNEL_MODULES_SUFFIX='\"${BCC_KERNEL_MODULES_SUFFIX}\"'")
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DKERNEL_HAS_SOURCE_DIR=${BCC_KERNEL_HAS_SOURCE_DIR}")
 add_library(clang_frontend loader.cc b_frontend_action.cc kbuild_helper.cc)

src/cc/frontends/clang/kbuild_helper.h

@@ -19,8 +19,6 @@
 #include <vector>
 #include <unistd.h>
 
-#define KERNEL_MODULES_DIR "/lib/modules"
-
 namespace ebpf {
 
 struct FileDeleter {