Fix doc build for 2 libraries which use own css.

I changed the default to use doc/src/boostbook.css. So add an explicit
location to libraries which use their own stylesheet.

[SVN r64170]

libs/numeric/conversion

@@ -1 +1 @@
-Subproject commit fe862ee762b63f8af72859e54c337d6bca9762e1
+Subproject commit ca4de5c31dea042feb3a6b68710ac34a291973e6

libs/optional

@@ -1 +1 @@
-Subproject commit 97e314f03adf256139e5ff85dcd7a17932afb059
+Subproject commit 57c07c7a57281a1094b44590e846b4ed1d5d3be3