fix libosmocore build for NuttX target
this fixes some compilation issues with libosmocore under NuttX,
particularly as some #defines are missing or some header files are
slightly different.
diff --git a/configure.ac b/configure.ac
index ef74932..acd153d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -31,7 +31,7 @@
dnl checks for header files
AC_HEADER_STDC
-AC_CHECK_HEADERS(execinfo.h sys/select.h sys/socket.h syslog.h ctype.h)
+AC_CHECK_HEADERS(execinfo.h sys/select.h sys/socket.h syslog.h ctype.h netinet/tcp.h)
# for src/conv.c
AC_FUNC_ALLOCA
AC_SEARCH_LIBS([dlopen], [dl dld], [LIBRARY_DL="$LIBS";LIBS=""])