ci build alpine: add workaround for msgpack-c to resolve relative paths

Author: otegami

alpine/build.sh

@@ -37,6 +37,7 @@ make -j$(nproc)
 make install
 cd -
 
+sed -i.bak -E -e 's,^(include|lib)dir=,\1dir=${prefix}/,g' /usr/lib/pkgconfig/msgpack-c.pc
 wget https://packages.groonga.org/source/groonga/groonga-${GROONGA_VERSION}.tar.gz
 tar xf groonga-${GROONGA_VERSION}.tar.gz
 cd groonga-${GROONGA_VERSION}