bug: 0.5.13 regression (bisected)

Hi! I've gotten a new build failure with gcc 15 (using git source) with 
dash 0.5.13. This is unrelated to the previously reported regression as 
our 0.5.13 has the patch that was provided to fix that one.

This build fails during the configure step when the script is ran with 
dash as `/usr/bin/sh`

```

# ../configure --build=x86_64-aerynos-linux --host=x86_64-aerynos-linux 
--prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib --disable-libssp 
--enable-cet --enable-checking=release --enable-default-pie 
--enable-default-ssp --enable-gnu-indirect-function 
--enable-host-bind-now --enable-host-pie --enable-link-serialization 
--enable-linker-build-id --enable-multilib --enable-plugin 
--enable-shared --enable-threads=yes --with-arch=x86-64-v2 
--with-arch_32=x86-64-v2 
--with-bugurl=https://github.com/AerynOS/recipes/issues 
--with-build-config=bootstrap-O1 --with-gcc-major-version-only 
--with-gxx-libcxx-include-dir=/usr/include/c++/v1 --with-isl 
--with-linker-hash-style=gnu --with-multilib-list=m32,m64 
--with-pkgversion=AerynOS --with-system-zlib --with-tune=ivybridge 
--with-tune_32=ivybridge --enable-bootstrap 
--enable-languages=c,c++,cobol,fortran,rust
checking build system type... x86_64-aerynos-linux-gnu
checking host system type... x86_64-aerynos-linux-gnu
checking target system type... x86_64-aerynos-linux-gnu
checking for a BSD-compatible install... /usr/bin/install -c
checking whether ln works... yes
checking whether ln -s works... yes
checking for a sed that does not truncate output... /usr/bin/sed
checking for gawk... gawk
checking for libatomic support... yes
checking for libitm support... yes
checking for libsanitizer support... yes
checking for libvtv support... yes
checking for libphobos support... yes
checking for libgcobol support... yes
checking for x86_64-aerynos-linux-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
checking for gcc option to accept ISO C89... none needed
checking for gcc option to accept ISO C99... none needed
checking whether we are using the GNU C++ compiler... yes
checking whether g++ accepts -g... yes
checking whether g++ accepts -static-libstdc++ -static-libgcc... no
checking for x86_64-aerynos-linux-gnatbind... no
checking for gnatbind... no
checking for x86_64-aerynos-linux-gnatmake... no
checking for gnatmake... no
checking whether compiler driver understands Ada and is recent enough... no
checking for x86_64-aerynos-linux-gdc... no
checking for gdc... no
checking whether the D compiler works... no
checking for cargo... cargo
checking how to compare bootstrapped objects... cmp --ignore-initial=16 
$$f1 $$f2
checking for objdir... .libs
checking for the correct version of gmp.h... yes
checking for the correct version of mpfr.h... yes
checking for the correct version of mpc.h... yes
checking for the correct version of the gmp/mpfr libraries... yes
checking for the correct version of the mpc libraries... yes
checking for isl 0.15 or later... yes
checking whether to enable libgdiagnostics... no
checking for library containing dlopen... none required
checking for library containing pthread_create... none required
../configure: 9961: .: cannot open ../gcc/cp/config-lang.in���M�U: No 
such file
```

I've bisected this failure to a9012f4078be32012f240c6208c3df53db3b6086, 
but unfortunately I'm not able to test 0.5.13 with that reverted as it 
has non-trivial merge conflicts (well, non-trivial to me at least).

Let me know if you have any further questions or troubleshooting steps 
you'd like me to try.

Regards,

Reilly