build: avoid unnecessary capnp-rpc dependency for mpgen #327

Instead, use only the required kj-async.

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ACK fa47449afe1405df661364ff22911892beb79715.

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

Code review ACK fa47449afe1405df661364ff22911892beb79715. Nice fix for the makefile, even if it doesn't solve netbsd