I'm using FreeBSD 13.0-CURRENT (for more information see here), and I want to use GNU m4. Downloaded this version of it, configured, and tried to make. But it gives error as stated in the title: error: don't know how to make ../../build-aux/snippet/c++defs.h. For the whole log, see here. Any idea? Thanks to all suggestions and answers.
EDIT1: indeed, my m4 version is 1.4.17.
Just unpacked m4-1.4.17 here (Fedora 31; your latest might be another version... but it seems to date from 2013). The offending file is there (in build-aux/snippet). There is a script called bootstrapincluded, but that presumably is only needed for sources straight from version control.
The traditional ./configure; make dance goes fine, but fails after compiling a bunch of stuff with:
freadahead.c: In function 'freadahead':
freadahead.c:91:3: error: #error "Please port gnulib freadahead.c to your platform! Look at the definition of fflush, fread, ungetc on your system, then report this to bug-gnulib."
A simple search for "GNU m4 FreeBSD" leads to FreshPorts, FreeBSD's m4 manual talks about a -g option to activate GNU m4 compatitility. Why aren't those enough? Presumably whatever patched version FreshPorts carries is a better starting point (if they are civilized and carry original sources and separate patches, porting the patches to another version is less work than debugging this mess yourself).
pkg search m4 yields amongst others: m4-1.4.18_1,1 GNU M4. I suspect pkg install m4 might well solve the OP's needs.