build: avoid unnecessary capnp-rpc dependency for mpgen

[maflcko]

Instead, use only the required kj-async.

[DrahtBot]

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

Reviews

See the guideline and AI policy for information on the review process.

Type	Reviewers
ACK	hebasto, ryanofsky

If your review is incorrectly listed, please copy-paste <!--meta-tag:bot-skip--> into the comment that the bot should ignore.

[hebasto]

ACK fa47449.

Possibly fixes #326

Unfortunately, this doesn't fix it:

$ cmake --build build -j 1
[ 14%] Building CXX object CMakeFiles/mputil.dir/src/mp/util.cpp.o
[ 14%] Built target mputil
[ 28%] Compiling Cap'n Proto schema include/mp/proxy.capnp
[ 42%] Building CXX object CMakeFiles/multiprocess.dir/src/mp/proxy.cpp.o
[ 57%] Building CXX object CMakeFiles/multiprocess.dir/include/mp/proxy.capnp.c++.o
[ 71%] Linking CXX static library libmultiprocess.a
[ 71%] Built target multiprocess
[ 85%] Building CXX object CMakeFiles/mpgen.dir/src/mp/gen.cpp.o
[100%] Linking CXX executable mpgen
/usr/bin/ld: warning: libstdc++.so.9, needed by /usr/pkg/lib/libcapnp-1.4.0.so, may conflict with libstdc++.so.7
/usr/bin/ld: warning: libstdc++.so.9, needed by /usr/pkg/lib/libcapnp-1.4.0.so, may conflict with libstdc++.so.7
/usr/bin/ld: CMakeFiles/mpgen.dir/src/mp/gen.cpp.o: in function `kj::Maybe<kj::Exception> kj::runCatchingExceptions<main::{lambda()#1}>(main::{lambda()#1}&&)':
gen.cpp:(.text+0x62f7): undefined reference to `kj::getCaughtExceptionAsKj()'
collect2: error: ld returned 1 exit status
*** [mpgen] Error code 1

make[2]: stopped making "CMakeFiles/mpgen.dir/build" in /home/hebasto/dev/libmultiprocess/build
make[2]: 1 error

make[2]: stopped making "CMakeFiles/mpgen.dir/build" in /home/hebasto/dev/libmultiprocess/build
*** [CMakeFiles/mpgen.dir/all] Error code 2

make[1]: stopped making "all" in /home/hebasto/dev/libmultiprocess/build
make[1]: 1 error

make[1]: stopped making "all" in /home/hebasto/dev/libmultiprocess/build
*** [all] Error code 2

make: stopped making "default_target" in /home/hebasto/dev/libmultiprocess/build
make: 1 error

make: stopped making "default_target" in /home/hebasto/dev/libmultiprocess/build

[ryanofsky]

Code review ACK fa47449. Nice fix for the makefile, even if it doesn't solve netbsd