I tested the --with-system-libmpdec successfully on my system. Most vendors are using the internal copy of libmpdec any way. AFAIK only Debian-based systems use their own system libmpdec.

$ ./configure -C --with-system-libmpdec
$ make
...
building '_decimal' extension
gcc -pthread -fPIC -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -std=c99 -Wextra -Wno-unused-parameter -Wno-missing-field-initializers -Werror=implicit-function-declaration -fvisibility=hidden -I./Include/internal -I./Include -I. -I/usr/local/include -I/home/heimes/dev/python/cpython/Include -I/home/heimes/dev/python/cpython -c /home/heimes/dev/python/cpython/Modules/_decimal/_decimal.c -o build/temp.linux-x86_64-3.11/home/heimes/dev/python/cpython/Modules/_decimal/_decimal.o -DCONFIG_64=1 -DASM=1
gcc -pthread -shared build/temp.linux-x86_64-3.11/home/heimes/dev/python/cpython/Modules/_decimal/_decimal.o -L/usr/local/lib -o build/lib.linux-x86_64-3.11/_decimal.cpython-311-x86_64-linux-gnu.so -lmpdec
...
$ ldd build/lib.linux-x86_64-3.11/_decimal.cpython-311-x86_64-linux-gnu.so 
        linux-vdso.so.1 (0x00007ffde21e1000)
        libmpdec.so.3 => /lib64/libmpdec.so.3 (0x00007f4f3b4cf000)
        libpthread.so.0 => /lib64/libpthread.so.0 (0x00007f4f3b4ae000)
        libc.so.6 => /lib64/libc.so.6 (0x00007f4f3b2df000)
        libm.so.6 => /lib64/libm.so.6 (0x00007f4f3b19b000)
        /lib64/ld-linux-x86-64.so.2 (0x00007f4f3b554000)