Do not pass -D_REENTRANT

This flag is not needed to build this library, and prevents to build
it with toolchains that don't have thread support.

Since the Makefile.in change is a one-liner, we also do it in the
patch to avoid having to autoreconfigure the package.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/Makefile.am
===================================================================
--- a/Makefile.am
+++ b/Makefile.am
@@ -1,4 +1,4 @@
-AM_CFLAGS = -Wall -Wwrite-strings -Werror -std=gnu99 -D_GNU_SOURCE -D_REENTRANT
+AM_CFLAGS = -Wall -Wwrite-strings -Werror -std=gnu99 -D_GNU_SOURCE
 
 EXTRA_DIST = README.html README-WIN32.html config.h.win32 doc
 
Index: b/Makefile.in
===================================================================
--- a/Makefile.in
+++ b/Makefile.in
@@ -219,7 +219,7 @@
 top_build_prefix = @top_build_prefix@
 top_builddir = @top_builddir@
 top_srcdir = @top_srcdir@
-AM_CFLAGS = -Wall -Wwrite-strings -Werror -std=gnu99 -D_GNU_SOURCE -D_REENTRANT
+AM_CFLAGS = -Wall -Wwrite-strings -Werror -std=gnu99 -D_GNU_SOURCE
 EXTRA_DIST = README.html README-WIN32.html config.h.win32 doc
 lib_LTLIBRARIES = libjson.la
 pkgconfigdir = $(libdir)/pkgconfig