Improve library finding + custom profile name + updated readme

Improves library finding by pretending to compile a c file (dummy.c
must exist).

Now that rust-lang/cargo#6988 is in stable, use "colcon" as a custom
profile name. Closes #2.

Use of IDL_PACKAGE_FILTER exemplified in README.md. Closes #3

.gitignore

@@ -1 +1,6 @@
 r2r_minimal_node/target
+
+# colcon stuff
+build/
+install/
+log/

README.md

@@ -24,10 +24,14 @@ cargo clean can be invoked by passing `-DCARGO_CLEAN=ON` to cmake, eg.
 colcon build --cmake-args -DCARGO_CLEAN=ON
 ```
 
+To keep builds performed by colcon separate, `r2r_cargo` *expects a custom profile called `colcon` to exist*. See <https://github.com/m-dahl/r2r_minimal_node/blob/master/r2r_minimal_node/Cargo.toml> for an example.
+
 ## Building using only cargo
 
 Another option is to build the message package first, then sourcing the resulting workspace. When `r2r_minimal_node_msgs` is sourced, the r2r build script will automatically pick up the custom messages (it defaults to building everything).
 
+To avoid building everything, it is possible to declare only the messages needed using the environment variable `IDL_PACKAGE_FILTER`. Setting this can be done in `.cargo/config.toml` for convenience, e.g. <https://github.com/m-dahl/r2r_minimal_node/blob/master/r2r_minimal_node/.cargo/config.toml>
+
 ``` sh
 mkdir src
 cd src

r2r_minimal_node/CMakeLists.txt

@@ -14,13 +14,11 @@ r2r_cargo(std_msgs               # just to test that it works
           rcl                    # we need the c ros2 api
           rcl_action             # as of r2r 0.1.0, we also need the action api
           r2r_minimal_node_msgs  # our custom message types
-          rmw_fastrtps_cpp       # (needed to build with RMW_IMPLEMENTATION=rmw_fastrtps_cpp)
-          FastRTPS               # (needed to build with RMW_IMPLEMENTATION=rmw_fastrtps_cpp)
          )
 
 # install binaries
 install(PROGRAMS
-  ${CMAKE_SOURCE_DIR}/target/release/${PROJECT_NAME}
+  ${CMAKE_SOURCE_DIR}/target/colcon/${PROJECT_NAME}
   DESTINATION lib/${PROJECT_NAME}
 )
 

r2r_minimal_node/Cargo.toml

@@ -4,7 +4,12 @@ version = "0.1.0"
 authors = ["Martin Dahl <martin.dahl@gmail.com>"]
 edition = "2021"
 
+# We use a custom profile to keep colcon
+# separate from "normal" rust building.
+[profile.colcon]
+inherits = "release"
+
 [dependencies]
-r2r = "0.6.3"
+r2r = "0.6.4"
 futures = "0.3.15"
 tokio = { version = "1", features = ["full"] }