Make it possible to add compiler options to a different variable than
CFLAGS. This is useful to conditionally disable a compiler warning for a
subpart of a tree.

We commonly only allow tabs at the beginning of a line, not
afterwards. The reason for this style is so that the code
looks formated right with tabstop=4 and tabstop=8.

This fixes the GCC 8 build. It disables the warning conditionally so that we
get the warning back if glib gets fixed.

It was disabled for shady reasons (not checking write() return value)
that are long gone. Worse even, it hid some real bugs.

We don't use them, so add a compiler warning about their use.
I think they are annoying, because sizeof(x) and typeof(x) might
evaluate at runtime. Especially the typeof() extension is useful
for macros, but behaves badly with variable-length arrays due to
running code at runtime.

But the worst is, G_STATIC_ASSERT() is implemented by declaring
an array of negative length. Usually, the checked condition should
be a compile time constant, but with VLAs the compiler would not evaluate
the static-assert at compile time and instead accept it silently. It's easy
to mess up static asserts to wrongly have a non-constant condition,
especially when the static-assert is used inside a macro.

Just say no.

Ideally we should switch to the standard macro AX_CODE_COVERAGE for
code coverage, but infortunately it's not supported on older distros
(e.g. on Travis).

Keep using the current macro, but remove the lcov version check as
it's not clear why it's needed in the first place (see [1]). The
latest version of lcov is now 1.13.

[1] http://git.savannah.gnu.org/gitweb/?p=autoconf-archive.git;a=commit;h=dedd290bdb647704d74c13992bd04275d9cb648e

  src/NetworkManagerUtils.c:347:18: error: suggest braces around initialization of subobject [-Werror,-Wmissing-braces]
          NMIPAddr a1 = { 0 }, a2 = { 0 };
                          ^
                          {}

Should we initialize unions this way? I think it's all right -- the initializer
works well and { { { 0 } } } is probably not what we'd like to see.

(cherry picked from commit 43012156a31fb452ba774196baf106027c2c9067)

... and prepend the $ld_gc_flags instead of appending to the $CFLAGS.