a little update to the sofia-sip library

git-svn-id: http://svn.freeswitch.org/svn/freeswitch/trunk@4939 d0543943-73ff-0310-b7d9-9358b9ac24b2

libs/sofia-sip/docs/build_system.txt

@@ -38,6 +38,11 @@ Makefile target notes
   separately in ``DIST_SOURCES`` variable (otherwise ``make dist``
   will fail)
 
+Makefile fragments
+------------------
+
+Some common makefile rules are in 'rules' subdirectory.
+
 Maintainer mode
 ---------------
 
@@ -54,13 +59,16 @@ option '--enable-maintainer-mode'.
 Running tests
 =============
 
-Sofia-SIP has quite complete suite of test cases. It is prudent to
-run them while making changes and before committing them to revision
-control system. However, running certain tests takes quite a long
-time. Therefore, they are run only if the environment variable
+Sofia-SIP has quite complete suite of test cases. It is prudent to run
+them while making changes and before committing them to revision control
+system. However, running certain tests takes quite a long time to
+execture. Therefore, they are run only if the environment variable
 EXPENSIVE_CHECKS has been set. EXPENSIVE_CHECKS is also set by the build
 system if configure option '--enable-expensive-checks' has been used.
 
+On hosts with i386 architecture, it is possible to run tests under
+valgrind. Use the make target 'valcheck' for that purpose.
+
 Code-tree layout
 ================