bug#62482: Unclear configure errors about "gl_PROG_CC_C99" and "gl_EARLY"

[c . buhtz]

Hello,

beside I'm interested into how to solve that problem (what is IMHO 
off-topic on that list) this message main intention is a bug report to 
improve the output of a configure script.

I have this two errors (?) running "configure". It is unclear what they 
are mean. Even if I look into the "configure" script the meaning is 
unclear to me.

    ./configure: line 5063: gl_PROG_CC_C99: command not found
    ./configure: line 5196: gl_EARLY: command not found

Please improve the output of the script to make it easier to understand 
it for the users, to prevent further support questions and lower the 
workload for emacs "support channels".

To reproduce:

I cloned from git and switch to "emacs-29" branch. The machine is a 
Raspberry Pi4 with Debian 11 (pure Debian, not rasbian).

Because the configure script is missing (bug-report exists) I checked 
the content of "configure.ac". It points out to use "autoconf" to 
generate such a script. I did.

$ autoconf
configure.ac:55: error: possibly undefined macro: AS_ECHO
      If this token and others are legitimate, please use 
m4_pattern_allow.
      See the Autoconf documentation.
configure.ac:1777: error: possibly undefined macro: m4_default

Two errors are there. I don't understand. But a "./configure" was 
created. So I tried.

$ ./configure
checking for xcrun... no
checking for GNU Make... make
checking build system type... aarch64-unknown-linux-gnu
checking host system type... aarch64-unknown-linux-gnu
checking for gcc... gcc
checking whether the C compiler works... yes
checking for C compiler default output file name... a.out
checking for suffix of executables...
checking whether we are cross compiling... no
checking for suffix of object files... o
checking whether we are using the GNU C compiler... yes
checking whether gcc accepts -g... yes
./configure: line 5063: gl_PROG_CC_C99: command not found
checking whether gcc and cc understand -c and -o together... yes
./configure: line 5196: gl_EARLY: command not found
checking whether gcc accepts -g3 -O2... yes
checking whether the compiler is clang... no
checking how to run the C preprocessor... gcc -E
./configure: line 5555: syntax error near unexpected token `-Wno-switch'
./configure: line 5555: `     gl_WARN_ADD(-Wno-switch)'