See here, why this should be always enabled:
https://lists.gnu.org/archive/html/autoconf-archive-maintainers/2016-06/msg00001.html
Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
--- /dev/null
+enable _REENTRANT when -lpthread is used
+
+Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
+
+diff -Nur gcc-6.2.0.orig/gcc/config/bfin/linux.h gcc-6.2.0/gcc/config/bfin/linux.h
+--- gcc-6.2.0.orig/gcc/config/bfin/linux.h 2016-01-04 15:30:50.000000000 +0100
++++ gcc-6.2.0/gcc/config/bfin/linux.h 2016-09-30 20:48:17.446636819 +0200
+@@ -38,6 +38,9 @@
+ "%{static:--start-group} %{mfast-fp:-lbffastfp} %G %L %{static:--end-group} \
+ %{!static:%{mfast-fp:-lbffastfp} %G}"
+
++#undef CPP_SPEC
++#define CPP_SPEC "%{posix:-D_POSIX_SOURCE} %{pthread:-D_REENTRANT}"
++
+ #undef LINK_SPEC
+ #define LINK_SPEC "\
+ %{mfdpic: -m elf32bfinfd -z text} %{shared} %{pie} \