SUMMARY
There is a syntax error in the "if amd64 assembler knows the MPX instructions" check.
The assembler never gets to see it.

STEPS TO REPRODUCE
1. On amd64, follow the build instructions from README, up through step 4

OBSERVED RESULT
The configure output includes the line:
checking if amd64 assembler knows the MPX instructions... no

EXPECTED RESULT
With a sufficiently-new assembler, it should instead be:
checking if amd64 assembler knows the MPX instructions... yes

SOFTWARE/OS VERSIONS
gcc (Debian 10.2.1-6) 10.2.1 20210110
GNU assembler (GNU Binutils for Debian) 2.35.2

ADDITIONAL INFORMATION
The relevant portion of config.log is:
configure:12750: checking if amd64 assembler knows the MPX instructions
configure:12773: gcc -c   conftest.c >&5
conftest.c: In function 'main':
conftest.c:51:11: error: expected ':' or ')' before 'bnd'
   51 |           bnd jmp  end\n"
      |           ^~~
conftest.c:51:23: error: stray '\' in program
   51 |           bnd jmp  end\n"
      |                       ^
conftest.c:51:25: warning: missing terminating " character
   51 |           bnd jmp  end\n"
      |                         ^
conftest.c:51:25: error: missing terminating " character
conftest.c:52:23: error: stray '\' in program
   52 |           foo: bnd ret\n"
      |                       ^
conftest.c:52:25: warning: missing terminating " character
   52 |           foo: bnd ret\n"
      |                         ^
conftest.c:52:25: error: missing terminating " character
conftest.c:53:19: warning: missing terminating " character
   53 |           end: nop");
      |                   ^
conftest.c:53:19: error: missing terminating " character
   53 |           end: nop");
      |                   ^~~
configure:12773: $? = 1

However, it's actually just missing some starting " characters.
See attached patch. The test even passes!