common: compile libstring-utils as C++

common: compile libstring-utils as C++

The code of string-utils.cpp is compiled as C++, but the functions are
still exported as C symbols for the moment (until all users are
converted to C++).

The only thing of interest here is this error:

      CXX      string-utils.lo
    /home/simark/src/lttng-tools/src/common/string-utils/string-utils.cpp: In function ‘star_glob_pattern_type_flags strutils_test_glob_pattern(const char*)’:
    /home/simark/src/lttng-tools/src/common/string-utils/string-utils.cpp:89:37: error: invalid conversion from ‘int’ to ‘star_glob_pattern_type_flags’ [-fpermissive]
       89 |                                 ret |= STAR_GLOB_PATTERN_TYPE_FLAG_END_ONLY;
          |                                 ~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          |                                     |
          |                                     int

In C++, you can't freely use bitwise operator on enumerators.  I added
an operator|= free function to handle it, which converts to the
underlying type and back.  If we have many of these enums used as flags,
we could think of adding a class for that, like enum_flags in GDB:

https://gitlab.com/gnutools/binutils-gdb/-/blob/7a6cb96b710257a4f5bc7e85cc103b6bf8dfc25c/gdbsupport/enum-flags.h

Change-Id: I64b458a6f6c1e5a131525826a116607eef824aaa
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>