build.jam: fix b2 install so cmake config is generated for openmethod

Two problems prevented `b2 install` from producing
`lib/cmake/boost_openmethod-<version>/boost_openmethod-config.cmake`,
breaking downstream consumers that use `find_package(Boost CONFIG
REQUIRED COMPONENTS openmethod)` (and the unified-cmake CI job):

1. The `boost_openmethod` alias used `usage-requirements` as a literal
   source name instead of the 5th argument position, and lived outside
   the `explicit` block. Move it inside `explicit` and put
   `<include>include <library>$(boost_dependencies)` in the
   usage-requirements slot, matching `assert`, `core`, `mp11`,
   `preprocessor`, etc.

2. The `call-if : boost-library openmethod` invocation had no `install`
   option, so the Jamroot fallback path declared empty install targets
   and `install-cmake-config` was called with no library names — no
   config files were generated. Add `: install boost_openmethod` so the
   library name reaches `install-or-stage-cmake-config`.

Verified locally on Windows: `b2 install` now produces
`lib/cmake/boost_openmethod-1.92.0/boost_openmethod-config.cmake`
(and the matching `-config-version.cmake`) defining `Boost::openmethod`.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: jll63

build.jam

@@ -15,12 +15,12 @@ constant boost_dependencies
 
 project /boost/openmethod ;
 
-alias boost_openmethod
-    : usage-requirements
-      <include>include
-      <library>$(boost_dependencies)
+explicit
+    [ alias boost_openmethod : : : : <include>include <library>$(boost_dependencies) ]
+    [ alias all : boost_openmethod test ]
     ;
-alias all : boost_openmethod test ;
-explicit all ;
 
-call-if : boost-library openmethod ;
+call-if
+    : boost-library openmethod
+    : install boost_openmethod
+    ;