Fix compilation on TruffleRuby [if libnlutils is installed into /usr/local/lib?]

[mike-bourgeous]

TruffleRuby's extension build process doesn't seem to check /etc/ld.so.conf, or for some other reason is not finding libraries in /usr/local/lib. Or, it could be that Mkmf's library search is generating code that fails to compile with TruffleRuby.

Errors building the gem in TruffleRuby:

conftest.c:14:57: error: use of undeclared identifier 'nl_unescape_string'
int t(void) { void ((*volatile p)()); p = (void ((*)()))nl_unescape_string; return !p; }
                                                        ^
1 error generated.
Process failed: #<Process::Status: pid 47735 exit 1>
checked program was:
/* begin */
 1: #include "ruby.h"
 2: 
 3: /*top*/
 4: extern int t(void);
 5: int main(int argc, char **argv)
 6: {
 7:   if (argc > 1000000) {
 8:     int (* volatile tp)(void)=(int (*)(void))&t;
 9:     printf("%d", (*tp)());
10:   }
11: 
12:   return !!argv[argc];
13: }
14: int t(void) { void ((*volatile p)()); p = (void ((*)()))nl_unescape_string; return !p; }
/* end */

ld.lld: error: unable to find library -lnlutils
clang-12: error: linker command failed with exit code 1 (use -v to see invocation)
Process failed: #<Process::Status: pid 47739 exit 1>
checked program was:
/* begin */
 1: #include "ruby.h"
 2: 
 3: /*top*/
 4: extern int t(void);
 5: int main(int argc, char **argv)
 6: {
 7:   if (argc > 1000000) {
 8:     int (* volatile tp)(void)=(int (*)(void))&t;
 9:     printf("%d", (*tp)());
10:   }
11: 
12:   return !!argv[argc];
13: }
14: extern void nl_unescape_string();
15: int t(void) { nl_unescape_string(); return 0; }
/* end */