Support distros that do not use FHS

[RossSmyth]

Not all distros place libm, libc, and libpthreads in lib. The typical ones are Guix, NixOs, and GoboLinux.

Currently these are hardcoded, meaning that mir cannot run on these distros without patching. I currently have been able to patch it pretty easily to find the .so files, but I'm not sure where the logic for finding system headers so, so I get many errors for "error in opening file stdio.h" and such when executing tests.

mir/mir-bin-run.c

Line 47 in 99c6507

static lib_t std_libs[]

mir/c2mir/c2mir-driver.c

Line 69 in 99c6507

static lib_t std_libs[]

mir/llvm2mir/llvm2mir-driver.c

Line 63 in 99c6507

} std_libs[] = {{"/lib64/libc.so.6", NULL}, {"/lib64/libm.so.6", NULL}};

mir/GNUmakefile

Line 134 in 99c6507

ifneq ($(shell test -f /usr/include/llvm-c/Core.h|echo 1), 1)