additional readme and comment notes

README.md

@@ -13,8 +13,9 @@ Ipopt.jl
 
 This will install Ipopt.jl, as well as Ipopt itself. A binary will be downloaded
 by default on macOS or Windows, and Ipopt will be automatically built from source
-on Linux. If your platform is not supported, or if you prefer to compile your own
-version of Ipopt in order to use commercial sparse linear algebra libraries, use
+on Linux unless a pre-existing version is found on the `LD_LIBRARY_PATH`.
+If your platform is not supported, or if you prefer to compile your own version
+of Ipopt in order to use commercial sparse linear algebra libraries, use
 the instructions below.
 
 **Custom Installation**:

deps/build.jl

@@ -2,6 +2,11 @@ using BinaryProvider
 using BinDeps
 using Compat
 
+# BinaryBuilder artifacts for linux currently assume libgfortran.so.4
+# is present, so they will not work for any source builds of Julia itself
+# when a GCC version older than 7 was used. BinaryBuilder also does not
+# support FreeBSD or non-glibc Linux distributions. Continue building from
+# source on non-apple unices while these limitations apply.
 use_BinaryProvider = Compat.Sys.isapple() || Compat.Sys.iswindows()
 
 @BinDeps.setup