make threadlocal use conditional on configure check. NB: This revision mixes GPLv2-only and GPLv3+ code so should be considered proof of concept.

configure.ac

@@ -40,6 +40,38 @@ AC_SUBST(XSEL_BIN)
 AC_SUBST(XSEL_MAN_PATH)
 
 
+# Copied from http://git.savannah.gnu.org/gitweb/?p=autoconf-archive.git;a=blob_plain;f=m4/ax_tls.m4, 
+# This definition is licenced under GPLv3+. 
+AC_DEFUN([AX_TLS], [
+  AC_MSG_CHECKING(for thread local storage (TLS) class)
+  AC_CACHE_VAL(ac_cv_tls, [
+    ax_tls_keywords="__thread __declspec(thread) none"
+    for ax_tls_keyword in $ax_tls_keywords; do
+       AS_CASE([$ax_tls_keyword],
+          [none], [ac_cv_tls=none ; break],
+          [AC_TRY_COMPILE(
+              [#include <stdlib.h>
+               static void
+               foo(void) {
+               static ] $ax_tls_keyword [ int bar;
+               exit(1);
+               }],
+               [],
+               [ac_cv_tls=$ax_tls_keyword ; break],
+               ac_cv_tls=none
+           )])
+    done
+  ])
+  AC_MSG_RESULT($ac_cv_tls)
+
+  AS_IF([test "$ac_cv_tls" != "none"],
+    AC_DEFINE_UNQUOTED([TLS], $ac_cv_tls, [If the compiler supports a TLS storage class define it to that here])
+      m4_ifnblank([$1], [$1]),
+    m4_ifnblank([$2], [$2])
+  )
+])
+
+AX_TLS
 
 #
 # If needed, run autoconf to regenerate the configure file

reader.cpp

@@ -184,7 +184,9 @@ commence.
 static volatile unsigned int s_generation_count;
 
 /* This threadlocal generation count is set when an autosuggestion background thread starts up, so it can easily check if the work it is doing is no longer useful. */
-static __thread unsigned int thread_generation_count;
+#ifdef TLS
+static TLS unsigned int thread_generation_count;
+#endif
 
 /* A color is an int */
 typedef int color_t;
@@ -672,8 +674,12 @@ int reader_reading_interrupted()
 
 bool reader_cancel_thread()
 {
+#ifdef TLS
     ASSERT_IS_BACKGROUND_THREAD();
     return s_generation_count != thread_generation_count;
+#else
+    return 0;
+#endif
 }
 
 void reader_write_title()
@@ -1234,7 +1240,9 @@ struct autosuggestion_context_t
             return 0;
         }
 
+#ifdef TLS
         thread_generation_count = generation_count;
+#endif
 
         /* Let's make sure we aren't using the empty string */
         if (search_string.empty())