iperf: fix out of tree configuration

The configure step uses a package header to check for support of types/declares,
this package header encapsulates all the required system header for providing
the needed resources. When configured from an out of tree directory the package
header is not found due to the hard path.
We now make the path for package header relative to our 'srcdir' so it is
found appropriately.

Signed-off-by: Awais Belal <awais_belal@mentor.com>
---
diff --git a/configure.ac b/configure.ac
index 2b3fd20..d420f9e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -176,8 +176,8 @@ dnl Check for system services
 
 dnl check for multicast
 if test "$ac_cv_multicast" != no; then
-  AC_CHECK_TYPES(struct ip_mreq,,,[#include "compat/headers_slim.h"])
-  AC_CHECK_DECLS(IP_ADD_MEMBERSHIP,,,[#include "compat/headers_slim.h"])
+  AC_CHECK_TYPES(struct ip_mreq,,,[#include "${srcdir}/compat/headers_slim.h"])
+  AC_CHECK_DECLS(IP_ADD_MEMBERSHIP,,,[#include "${srcdir}/compat/headers_slim.h"])
   AC_MSG_CHECKING(for multicast support)
   ac_cv_multicast=no
   if test "$ac_cv_have_decl_IP_ADD_MEMBERSHIP" = yes; then
@@ -193,9 +193,9 @@ fi
 
 dnl check for IPv6
 if test "$ac_cv_have_ipv6" != no; then
-  AC_CHECK_TYPES(struct sockaddr_storage,,,[#include "compat/headers_slim.h"])
-  AC_CHECK_TYPES(struct sockaddr_in6,,,[#include "compat/headers_slim.h"])
-  AC_CHECK_DECLS(AF_INET6,,,[#include "compat/headers_slim.h"])
+  AC_CHECK_TYPES(struct sockaddr_storage,,,[#include "${srcdir}/compat/headers_slim.h"])
+  AC_CHECK_TYPES(struct sockaddr_in6,,,[#include "${srcdir}/compat/headers_slim.h"])
+  AC_CHECK_DECLS(AF_INET6,,,[#include "${srcdir}/compat/headers_slim.h"])
   AC_MSG_CHECKING(for IPv6 headers and structures)
   ac_cv_have_ipv6=no
   if test "$ac_cv_type_struct_sockaddr_storage" = yes; then
@@ -211,9 +211,9 @@ fi
 
 if test "$ac_cv_have_ipv6" = yes; then
   if test "$ac_cv_multicast" = yes; then
-    AC_CHECK_TYPES(struct ipv6_mreq,,,[#include "compat/headers_slim.h"])
-    AC_CHECK_DECLS(IPV6_ADD_MEMBERSHIP,,,[#include "compat/headers_slim.h"])
-    AC_CHECK_DECLS(IPV6_MULTICAST_HOPS,,,[#include "compat/headers_slim.h"])
+    AC_CHECK_TYPES(struct ipv6_mreq,,,[#include "${srcdir}/compat/headers_slim.h"])
+    AC_CHECK_DECLS(IPV6_ADD_MEMBERSHIP,,,[#include "${srcdir}/compat/headers_slim.h"])
+    AC_CHECK_DECLS(IPV6_MULTICAST_HOPS,,,[#include "${srcdir}/compat/headers_slim.h"])
     AC_MSG_CHECKING(for IPv6 multicast support)
     ac_cv_have_ipv6_multicast=no
     if test "$ac_cv_type_struct_ipv6_mreq" = yes; then