Fix some compiler warning (iovisor#2047)

* Allow unused return value in cc source

With llvm-7.0.0,
some annoying warning messeges are raised:

	/home/lecopzer/workspace/bcc/src/cc/libbpf.c:456:3: warning: ignoring return value of ‘fgets’, declared with attribute warn_unused_result [-Wunused-result]
   	fgets(fmt, sizeof(fmt), f); // pos
   	^~~~~~~~~~~~~~~~~~~~~~~~~~

/home/lecopzer/workspace/bcc/src/cc/libbpf.c: In function ‘bpf_prog_get_tag’:
/home/lecopzer/workspace/bcc/src/cc/libbpf.c:456:3: warning: ignoring return value of ‘fgets’, declared with attribute warn_unused_result [-Wunused-result]
   fgets(fmt, sizeof(fmt), f); // pos
   ^~~~~~~~~~~~~~~~~~~~~~~~~~
...

/home/lecopzer/workspace/bcc/tests/cc/utils.cc: In function ‘int cmd_scanf(const char*, const char*, ...)’:
/home/lecopzer/workspace/bcc/tests/cc/utils.cc:30:10: warning: ignoring return value of ‘int vfscanf(FILE*, const char*, __va_list_tag*)’, declared with attribute warn_unused_result [-Wunused-result]
   vfscanf(pipe, fmt, args);
   ~~~~~~~^~~~~~~~~~~~~~~~~

Let get rid of them by adding -Wno-unused-result.

* cc: Fix comparison between signed and unsigned value

With llvm-7.0.0:

/home/lecopzer/workspace/bcc/src/cc/common.cc: In function ‘std::__cxx11::string ebpf::get_pid_exe(pid_t)’:
/home/lecopzer/workspace/bcc/src/cc/common.cc:60:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if (res >= sizeof(exe_path))
       ~~~~^~~~~~~~~~

As the declaration of `exe_path` is `char exe_path[4096]`,
the `sizeof(exe_path)` would always return 4096 (unsigned), so it's safe
to static cast to `int` unless it's larger than 2^31 - 1.

src/cc/CMakeLists.txt

@@ -15,7 +15,8 @@ add_definitions(${LLVM_DEFINITIONS})
 configure_file(libbcc.pc.in ${CMAKE_CURRENT_BINARY_DIR}/libbcc.pc @ONLY)
 
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fPIC -DBCC_PROG_TAG_DIR='\"${BCC_PROG_TAG_DIR}\"'")
-set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fPIC")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-unused-result")
+set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fPIC -Wno-unused-result")
 
 string(REGEX MATCH "^([0-9]+).*" _ ${LLVM_PACKAGE_VERSION})
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DLLVM_MAJOR_VERSION=${CMAKE_MATCH_1}")

src/cc/common.cc

@@ -57,7 +57,7 @@ std::string get_pid_exe(pid_t pid) {
  res = readlink(exe_link.c_str(), exe_path, sizeof(exe_path));
  if (res == -1)
  return "";
- if (res >= sizeof(exe_path))
+ if (res >= static_cast<int>(sizeof(exe_path)))
  res = sizeof(exe_path) - 1;
  exe_path[res] = '\0';
  return std::string(exe_path);

tests/cc/CMakeLists.txt

@@ -9,6 +9,9 @@ target_link_libraries(test_static bcc-static)
 
 add_test(NAME c_test_static COMMAND ${TEST_WRAPPER} c_test_static sudo ${CMAKE_CURRENT_BINARY_DIR}/test_static)
 
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-unused-result")
+set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wno-unused-result")
+
 if(ENABLE_USDT)
 add_executable(test_libbcc
  test_libbcc.cc