makecontext, swapcontext, … are not provided on musl as there are no
longer mandated by POSIX. Instead, these functions are available via a
separately library (libucontext), however, we need to link explicitly
against this.

diff -upr capnproto-c++-0.8.0.orig/src/kj/CMakeLists.txt capnproto-c++-0.8.0/src/kj/CMakeLists.txt
--- capnproto-c++-0.8.0.orig/src/kj/CMakeLists.txt	2021-02-04 19:39:00.666936799 +0100
+++ capnproto-c++-0.8.0/src/kj/CMakeLists.txt	2021-02-04 19:39:27.560397981 +0100
@@ -74,6 +74,7 @@ add_library(CapnProto::kj ALIAS kj)
 # TODO(cleanup): Use cxx_std_14 once it's safe to require cmake 3.8.
 target_compile_features(kj PUBLIC cxx_generic_lambdas)
 
+target_link_libraries(kj PUBLIC ucontext)
 if(UNIX AND NOT ANDROID)
   target_link_libraries(kj PUBLIC pthread)
 endif()