Add description of USE_SYSTEM_... flags to readme, especially explain…

…ing how to use a system SuiteSparse library. closes JuliaLang#1101

README.md

@@ -166,6 +166,8 @@ Julia uses the following external libraries, which are automatically downloaded
 [GLPK]: http:https://www.gnu.org/software/glpk/
 [Rmath]: http:https://cran.r-project.org/doc/manuals/R-admin.html#The-standalone-Rmath-library
 
+If you already have one or more of these packages installed on your system, it is possbile to pass `USE_SYSTEM_...=1` to `make` to prevent Julia from compiling duplicates of these libraries. The complete list of possible flags can be found in Make.inc (or pass `USE_DEBIAN=1` to make if you have all build dependancies and want the minimal Julia build). Please be aware that this proceedure is not officially supported, as it introduces additional variablity into the installation and versioning of the dependancies, and is recommended only for system package maintainers. Unexpected compile errors may result, as the build system will do no further checking to ensure the proper packages are installed. SuiteSparse is a special case, since it is typically only installed as a static library, however `USE_SYSTEM_SUITESPARSE=1` requires that it is a shared library; running the script `contrib/repackage_system_suitesparse4.make` will copy your static system SuiteSparse installation into the shared library format required by Julia.
+
 <a name="Directories"/>
 ## Directories
 
@@ -231,4 +233,4 @@ Julia has a web REPL with very preliminary graphics capabilities. The web REPL i
 ### Try it Online
 
 [Forio.com](http:https://forio.com/) is generously hosting and maintaining an instance of Julia's web REPL here: [julia.forio.com](http:https://julia.forio.com).
-This service is best-effort and may not always be up or stable. Be nice!
+This service is best-effort and may not always be up or stable. Be nice!