build: Vendor ax_code_coverage.m4

The newest version of this macro changes the way you include it, which is
a mess because we don't know which version a particular distro will have.
We copy the old version of the macro into the m4/ directory and include
it in git. That should prevent aclocal from installing the newer version.

(It would be better to include the newer version, but that has a bug that
clobbers the GITIGNORE variable.)

Closes #149
25 jobs for 149-vendor-ax-code-coverage in 19 minutes and 57 seconds (queued for 74 minutes and 26 seconds)
Status Job ID Name Coverage
  Source Check
passed #70367
cppcheck

00:00:50

passed #70368
cpplint

00:00:54

passed #70369
eslint

00:00:38

passed #70366
fedora

00:02:27

 
  Test
passed #70372
no_graphics

00:02:00

passed #70373
no_profiler

00:02:09

passed #70374
no_readline

00:02:10

passed #70370
sanitizer_gcc

00:04:00

passed #70371
with_logging

00:02:10

 
  Manual
manual #70388
allowed to fail manual
armv8
manual #70375
allowed to fail manual
codequality
manual #70376
allowed to fail manual
code_statistics
passed #70390
manual
coverage

00:02:33

manual #70378
allowed to fail manual
fedora_gcc
manual #70384
allowed to fail manual
flatpak
manual #70379
allowed to fail manual
installed_tests
manual #70382
allowed to fail manual
lts
manual #70389
allowed to fail manual
ppc64le
manual #70377
allowed to fail manual
sanitizer_clang
manual #70381
allowed to fail manual
ubuntu_clang
manual #70380
allowed to fail manual
ubuntu_gcc
manual #70383
allowed to fail manual
valgrind
manual #70387
allowed to fail manual
zeal_11
manual #70385
allowed to fail manual
zeal_2
manual #70386
allowed to fail manual
zeal_4