bug#44992: Use of uninitialized value $macro in exists at /usr/bin/automake line 5268, <GEN0> line 1

[HaRRy[LP]]

Hi,

I've got:

```

running: automake -a

Use of uninitialized value $macro in exists at /usr/bin/automake line 5268, <GEN0> line 1.
Use of uninitialized value $macro in concatenation (.) or string at /usr/bin/automake line 5268, <GEN0> line 1.
automake: error: unrequested trace ''

automake: Please contact <bug-automake@gnu.org>.
 at /usr/share/automake-1.16/Automake/Channels.pm line 662, <GEN0> line 1.
      Automake::Channels::msg("automake", "", "unrequested trace ''") called at /usr/share/automake-1.16/Automake/ChannelDefs.pm line 212
      Automake::ChannelDefs::prog_error("unrequested trace ''") called at /usr/bin/automake line 5268
      Automake::scan_autoconf_traces("configure.ac") called at /usr/bin/automake line 5527
      Automake::scan_autoconf_files() called at /usr/bin/automake line 8355

```

[As you can also see in my CI log: https://github.com/pmem/pmemkv/runs/1477402876?check_suite_focus=true#step:4:1997 ]

Steps to reproduce was as follows:

```

git clone https://github.com/pmem/valgrind.git  (but tested also with non-forked, original repo: git clone git://sourceware.org/git/valgrind.git)
cd valgrind
./autogen.sh
```
[this is a test step of my CI: https://github.com/pmem/pmemkv/blob/master/utils/docker/images/install-valgrind.sh#L31 )

Environment:

Fedora rawhide

autoconf           noarch  2.69-34.fc33

automake         noarch  1.16.2-3.fc34