document some gotchas for local build process

doc/build-local.md

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+# Building IDAES binaries locally
+
+For testing purposes, it may be useful to build the binaries locally, i.e. not
+using the Docker build scripts. This is also necessary for the MacOS builds
+(see the MacOS section in `build.md` for more information).
+
+You can build locally by running the following scripts:
+
+- `scripts/compile_solvers.sh`: Compile solvers (including `k_aug`, `dot_sens`,
+`petsc`, and all solver-associated libraries).
+- `scripts/compile_libs.sh`: Compile shared libraries for external functions.
+
+It is recommended to perform an "out-of-source" build. This can be done by
+first running `scripts/build_directory.sh _build` from the top level of this
+repository. This copies source code, makefiles, patch files, and build scripts
+into the `_build` directory, where you will actually run the build.
+Move into the build directory with `cd _build`.
+
+The build scripts were not designed for local use, so there are a few gotchas:
+
+- We look for patch files in a specific location relative to the original
+working directory. I.e. you must run `compile_solvers.sh` from *one level above*
+the scripts directory in order for patches (e.g. `ipopt.pc.patch`) to be
+correctly applied.
+- `ipopt.pc.patch` assumes that `coinmumps` and `coinhsl` are listed as
+dependencies in the `ipopt.pc` file. If either is not found (i.e. we are
+compiling without HSL), the patch application will fail.
+- By default, Petsc is assumed to be installed in hard-coded, OS-dependent
+location. If you are compiling locally, you have probably installed it
+in your own preferred location. To override the default, set the `PETSC_DIR`
+environment variable to the location where you have installed Petsc.
+- If you have your own system-installed HSL libraries that are discoverable
+by the linker (i.e. `LD_LIBRARY_PATH` is set), the COIN-OR solvers will likely
+be able to find and link against them. In this case, you will see a
+`HSL Present: NO` message (as there was no `coinhsl.zip` in the parent of the
+working directory), but will likely still build HSL-enabled solvers.