make sure libosmocom builds when cross-compiling for ARM without OS

This is required to build the library and be able to use it from
within OsmocomBB.
diff --git a/libosmocore/configure.in b/libosmocore/configure.in
index b5898bd..a5c5f6b 100644
--- a/libosmocore/configure.in
+++ b/libosmocore/configure.in
@@ -14,6 +14,7 @@
 
 dnl checks for header files
 AC_HEADER_STDC
+AC_CHECK_HEADERS(execinfo.h sys/select.h)
 
 # The following test is taken from WebKit's webkit.m4
 saved_CFLAGS="$CFLAGS"