libbb2: Use "--enable-fat=yes" to avoid build nondeterminism

Otherwise it would pick various -march flags based on the CPU of the
compiling system, using beautiful code like this:

````
 63 AC_CACHE_CHECK(for x86 cpuid $1 output, ax_cv_gcc_x86_cpuid_$1,
 64  [AC_RUN_IFELSE([AC_LANG_PROGRAM([#include <stdio.h>], [
 65      int op = $1, eax, ebx, ecx, edx;
 66      FILE *f;
 67       __asm__("cpuid"
 68         : "=a" (eax), "=b" (ebx), "=c" (ecx), "=d" (edx)
 69         : "a" (op));
 70      f = fopen("conftest_cpuid", "w"); if (!f) return 1;
 71      fprintf(f, "%x:%x:%x:%x\n", eax, ebx, ecx, edx);
 72      fclose(f);
 73      return 0;
 74 ])],

...

121       AC_CACHE_CHECK([whether avx is supported], [ax_cv_have_avx_ext],
122       [
123         ax_cv_have_avx_ext=no
124         if test "$((0x$ecx>>28&0x01))" = 1; then
125           ax_cv_have_avx_ext=yes
126         fi
127       ])
````