Missing <stdlib.h> include in lib/autoconf/c.m4

Hello,

When working on Clang 16 (and presumably GCC 14) porting [0], I noticed the following
when building GNU grep 3.8:
```
configure:6215: checking for clang-16 option to enable C11 features
configure:6230: clang-16  -c -g -O2  conftest.c >&5
conftest.c:26:14: warning: a function definition without a prototype is deprecated in all versions of C and is not supported in C2x [-Wdeprecated-non-prototype]
static char *e (p, i)
             ^
conftest.c:246:3: error: call to undeclared library function 'free' with type 'void (void *)'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
  free (ia);
  ^
conftest.c:246:3: note: include the header <stdlib.h> or explicitly provide a declaration for 'free'
1 warning and 1 error generated.
[...]
```

Tracing it through, it looks like there's a missing <stdlib.h> include for free() in autoconf's lib/autoconf/c.m4 (_AC_C_C99_TEST_GLOBALS).

[0] https://lwn.net/Articles/913505/
[1] https://discourse.llvm.org/t/configure-script-breakage-with-the-new-werror-implicit-function-declaration/65213

Best,
sam

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