Crashfix: CFLAGS -fsanitize=address could lead to crash if pbkdf2 mod loaded

[michaelortmann]

Found by: michaelortmann
Patch by: michaelortmann
Fixes:

One-line summary:
Compiling eggdrop with CFLAGS -fsanitize=address on systems that require linker flag -lresolv for b64_ntop() or b64_pton() could lead to crash if pbkdf2 mod loaded

Additional description (if needed):
gccs sanitizer will intercept calls to b64_ntop or b64_pton()
https://github.com/gcc-mirror/gcc/blob/releases/gcc-13/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc#L2512
which leads to eggdrops configure script misdetecting that eggdrop can be linked without -lresolv to call those functions.
Please run misc/runautotools after merging or for testing this PR

Test cases demonstrating functionality (if applicable):
Before:

$ CFLAGS="-fsanitize=address" ./configure
[...]
checking for b64_ntop... yes
[...]
.chpass testuser 123456
[03:15:50.%f] tcl: builtin dcc call: *dcc:chpass -HQ 1 [something]
[03:15:50.%f] * Last context: tclhash.c/726 [Tcl proc: *dcc:chpass, param:  $_dcc1 $_dcc2 $_dcc3]
[03:15:50.%f] * Please REPORT this BUG!
[03:15:50.%f] * Check doc/BUG-REPORT on how to do so.
[03:15:49.%f] * Wrote DEBUG
[03:15:49.%f] * SEGMENT VIOLATION -- CRASHING!
Segmentation fault (core dumped)
$ coredumpctl debug
[...]
#0  0x0000000000000000 in ?? ()
(gdb) bt full
#0  0x0000000000000000 in ?? ()
No symbol table info available.
#1  0x00007fe8abe81a91 in __interceptor___b64_ntop (src=0x7fe8a9003ce0 "m+r\032\036\303b\v*Y\373n\026\213\236O", 
    srclength=16, target=0x7fe8aa65573b <out+27> "", targsize=76)
    at /usr/src/debug/gcc/gcc/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:2512
        ctx = <optimized out>
        _ctx = <optimized out>
        res = <optimized out>
        v1 = <optimized out>
        v2 = <optimized out>
[...]

After:

$ CFLAGS="-fsanitize=address" ./configure
[...]
checking for b64_ntop... yes
[...]
checking for b64_ntop... no
checking for b64_ntop with -lresolv... yes
[...]
.chpass testuser 123456
[03:21:55.%f] tcl: builtin dcc call: *dcc:chpass -HQ 1 [something]
[03:21:55.%f] pbkdf2 method SHA256 rounds 1000, user 0.186ms sys 1.114ms
[03:21:55.%f] #-HQ# chpass testuser [something]
Changed password.

[thommey]

I'd rather see AC_RUN_IFELSE compile and run a test program that calls those functions to verify they're really available. Other flags could do the same and that'd be future-proof. We do this for ipv6 support, too.

[michaelortmann]

Ty, this idea makes the patch even better. Ready for review again.