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 ])
````
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