build: Vendor last-good version of AX_CODE_COVERAGE

It's not enough to copy the ax_code_coverage.m4 file; as explained by
Simon McVittie on gnome-hackers IRC, aclocal will still copy in a newer
version if its #serial number is higher than the existing one.

In order to prevent the macro from being overwritten with a broken
version, we rename the file and also rename the macro.

See #149 for more explanation.

(Unreviewed, pushing to fix build.)
26 jobs for test-m4 in 20 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Source Check
canceled #206724
cppcheck

00:00:19

canceled #206725
cpplint

00:00:19

canceled #206726
eslint

00:00:19

canceled #206723
fedora

00:00:20

 
  Test
canceled #206730
no_graphics

canceled #206731
no_profiler

canceled #206732
no_readline

canceled #206727
sanitizer_gcc

canceled #206728
with_logging

canceled #206729
with_systemtap

 
  Manual
canceled #206746
allowed to fail manual
armv8

canceled #206733
allowed to fail manual
codequality

canceled #206734
allowed to fail manual
code_statistics

canceled #206748
allowed to fail manual
coverage

canceled #206736
allowed to fail manual
fedora_gcc

canceled #206742
allowed to fail manual
flatpak

canceled #206737
allowed to fail manual
installed_tests

canceled #206740
allowed to fail manual
lts

canceled #206747
allowed to fail manual
ppc64le

canceled #206735
allowed to fail manual
sanitizer_clang

canceled #206739
allowed to fail manual
ubuntu_clang

canceled #206738
allowed to fail manual
ubuntu_gcc

canceled #206741
allowed to fail manual
valgrind

canceled #206745
allowed to fail manual
zeal_11

canceled #206743
allowed to fail manual
zeal_2

canceled #206744
allowed to fail manual
zeal_4