hugo: add note on boost components

hugo/content/installation/building/unix/third-party.md

@@ -17,9 +17,14 @@ Required software:
 * `libtiff >=4.0.2`
 * `libcerf >=1.14` (complex error function, maintained by us)
 * `python3, python3-devel, python3-numpy, python3-matplotlib`
-* `boost >= 1.65`
+* `boost >= 1.65` (see note below)
 * `Qt5 >=5.5.1` (required modules are listed below)
 
+From libboost, we need the library components `iostreams` and `program_options`.
+Internally, the CMake command to find `boost::iostreams` may add a dependence
+on `boost::regex`. Furthermore, we need a number of header-only components,
+including `algorithm/string`, `bimap`, `geometry`, `numeric/interval`, `polymorphic_cast`.
+
 Recommended software:
 
 * `perl` (for generating a man page)