build fails on AIX 7.1 due to thread-local storage

[infrastation]

Commit b10eefd seems to be the breaking change.

Using Autoconf and XL C 12.1:

$ make -s CFLAGS=-qhalt=w
"./pcap.c", line 3428.16: 1506-731 (S) The '__thread' keyword is not supported on the target platform.  The keyword is ignored.
"./pcap.c", line 3694.16: 1506-731 (S) The '__thread' keyword is not supported on the target platform.  The keyword is ignored.
"./etherent.c", line 92.16: 1506-731 (S) The '__thread' keyword is not supported on the target platform.  The keyword is ignored.
make: 1254-004 The error code from the last command is 1.

Using Autoconf and GCC 8.3.0:

$ make -s CFLAGS=-Werror
config.status: creating libpcap.pc.tmp
config.status: creating pcap-config.tmp
ld: 0711-317 ERROR: Undefined symbol: __get_tpointer
ld: 0711-345 Use the -bloadmap or -bnoquiet option to obtain more information.
collect2: error: ld returned 8 exit status
make: 1254-004 The error code from the last command is 1.

(The only description of __get_tpointer I managed to find is this and it seems to be 32-bit specific, which looks wrong.)

[infrastation]

Adding -lpthreads seems to fix the GCC part of the problem.

[infrastation]

Running XL C with -qtls replaces "The '__thread' keyword is not supported" with a linked error about __tls_get_mod(), which in turn resolves with -lpthreads. Let me prepare a bug fix...

[guyharris]

There'a already stuff for both autotools and CMake to check for libpthreads and to check whether it's necessary; the latter should be expanded to see whether using thread-local data requires libpthreads.

[infrastation]

This is what I was just going to commit:

--- a/aclocal.m4
+++ b/aclocal.m4
@@ -101,6 +101,23 @@ AC_DEFUN(AC_LBL_C_INIT,
 
            case "$host_os" in
 
+           aix*)
+                   case "$CC" in
+                   *xlc*)
+                       # XL C supports the "__thread" storage class specifier
+                       # and the associated "-qtls" command-line option since
+                       # at least version 12.1.0, the newer "-ftls-model"
+                       # option became available in version 16.1.0.  By
+                       # default TLS is enabled on Linux and disabled on AIX.
+                       #
+                       # However, XL C 17 is actually LLVM, which will require
+                       # a more sophisticated workaround if and when it is
+                       # available on AIX.
+                       $1="$$1 -qtls"
+                       ;;
+                   esac
+                   ;;
+
            darwin*)
                    #
                    # This is assumed either to be GCC or clang, both
diff --git a/configure.ac b/configure.ac
index cb93687e..5498ba45 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2079,6 +2079,9 @@ aix*)
                LIBS="-lodm -lcfg"
                ;;
        esac
+       # Thread-local storage on AIX requires libpthreads no matter if using
+       # GCC or XL C.
+       LIBS="$LIBS -lpthreads"
        ;;
 
 darwin*)

If you see a better solution, let me know and I will work on a different problem.