Remove mention of Intel compilers and caution against using SYSTEM libraries

Make.inc

@@ -23,7 +23,9 @@ OPENBLAS_DYNAMIC_ARCH:=1
 endif
 OPENBLAS_USE_THREAD:=1
 
-# Use libraries available on the system instead of building them
+# Flags for using libraries available on the system instead of building them.
+# Please read the notes around usage of SYSTEM flags in README.md
+# Issues resulting from use of SYSTEM versions will generally not be accepted.
 USE_SYSTEM_LLVM:=0
 USE_SYSTEM_LIBUNWIND:=0
 DISABLE_LIBUNWIND:=0

README.md

@@ -68,7 +68,7 @@ Julia is built and tested regularly on the following platforms:
 All systems marked with ✓ for CI are tested using continuous integration for every commit.
 Systems with ✓ for binaries have official binaries available on the [downloads](https://julialang.org/downloads) page and are tested regularly. The PTX backend needs a source build and the [CUDAnative.jl](https://github.com/JuliaGPU/CUDAnative.jl) package.
 The systems listed here with neither CI nor official binaries are known to build and work, but ongoing support for those platforms is dependent on community efforts.
-It's possible that Julia will build and work on other platforms too, and we're always looking to better our platform coverage.
+It is possible that Julia will build and work on other platforms too, and we're always looking to better our platform coverage.
 If you're using Julia on a platform not listed here, let us know!
 
 ## Source Download and Compilation
@@ -199,6 +199,7 @@ Julia does not install anything outside the directory it was cloned into. Julia
 * GCC version 4.7 or later is required to build Julia.
 * To use external shared libraries not in the system library search path, set `USE_SYSTEM_XXX=1` and `LDFLAGS=-Wl,-rpath,/path/to/dir/contains/libXXX.so` in `Make.user`.
   * Instead of setting `LDFLAGS`, putting the library directory into the environment variable `LD_LIBRARY_PATH` (at both compile and run time) also works.
+* The `USE_SYSTEM_*` flags should be used with caution. These are meant only for troubleshooting, porting, and packaging, where package maintainers work closely with the Julia developers to make sure that Julia is built correctly. Production use cases should use the officially provided binaries. Issues arising from the use of these flags will generally not be accepted.
 * See also the [external dependencies](#required-build-tools-and-external-libraries).
 
 #### Architecture Customization
@@ -250,7 +251,7 @@ The remaining build tools are available from the Ports Collection, and can be in
 To build Julia, simply run `gmake`.
 (Note that `gmake` must be used rather than `make`, since `make` on FreeBSD corresponds to the incompatible BSD Make rather than GNU Make.)
 
-It's important to note that the `USE_SYSTEM_*` flags should be used with caution on FreeBSD.
+As mentioned above, it is important to note that the `USE_SYSTEM_*` flags should be used with caution on FreeBSD.
 This is because many system libraries, and even libraries from the Ports Collection, link to the system's `libgcc_s.so.1`,
 or to another library which links to the system `libgcc_s`.
 This library declares its GCC version to be 4.6, which is too old to build Julia, and conflicts with other libraries when linking.
@@ -350,7 +351,7 @@ Julia uses the following external libraries, which are automatically downloaded
 
 ### Notes for distribution package maintainers
 
-Package maintainers will typically want to make use of system libraries where possible. Please refer to the above version requirements and additional notes below. A list of maintained Julia packages for various platforms is available at https://julialang.org/downloads/platform.html.
+We understand that package maintainers will typically want to make use of system libraries where possible. Please refer to the above version requirements and additional notes below. It is strongly advised that package maintainers apply the patches included in the Julia repo for LLVM and other libraries, should they choose to use SYSTEM versions. A list of maintained Julia packages for various platforms is available at https://julialang.org/downloads/platform.html.
 
 ### System Provided Libraries
 
@@ -377,15 +378,7 @@ Julia uses a custom fork of libuv. It is a small dependency, and can be safely b
 
 As a high-performance numerical language, Julia should be linked to a multi-threaded BLAS and LAPACK, such as OpenBLAS or ATLAS, which will provide much better performance than the reference `libblas` implementations which may be default on some systems.
 
-### SuiteSparse
-
-SuiteSparse is a special case, since it is typically only installed as a static library, while `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. `make USE_SYSTEM_SUITESPARSE=1` will then use the SuiteSparse that has been copied into Julia's directory, but will not build a new SuiteSparse library from scratch.
-
-### Intel compilers and Math Kernel Library (MKL)
-
-To build Julia using the Intel compilers (icc, icpc, ifort), and link against
-the [MKL] BLAS and LAPACK libraries, first make sure you have a recent version
-of the compiler suite (version 15 or later).
+### Intel MKL
 
 For a 64-bit architecture, the environment should be set up as follows:
 
@@ -394,8 +387,6 @@ For a 64-bit architecture, the environment should be set up as follows:
 
 Add the following to the `Make.user` file:
 
-    USEICC = 1
-    USEIFC = 1
     USE_INTEL_MKL = 1
     USE_INTEL_LIBM = 1