partial bitcoin#30940: Fix build with MULTIPROCESS=1 in Guix environment

includes:
- 06b4c33

Author: kwvg

depends/packages/capnp.mk

@@ -9,6 +9,7 @@ define $(package)_set_vars :=
   $(package)_config_opts := -DBUILD_TESTING=OFF
   $(package)_config_opts += -DWITH_OPENSSL=OFF
   $(package)_config_opts += -DWITH_ZLIB=OFF
+  $(package)_cxxflags += -ffile-prefix-map=$$($(package)_extract_dir)=/usr
 endef
 
 define $(package)_config_cmds

depends/packages/libmultiprocess.mk

@@ -13,6 +13,7 @@ ifneq ($(host),$(build))
 $(package)_config_opts := -DCAPNP_EXECUTABLE="$$(native_capnp_prefixbin)/capnp"
 $(package)_config_opts += -DCAPNPC_CXX_EXECUTABLE="$$(native_capnp_prefixbin)/capnpc-c++"
 endif
+$(package)_cxxflags += -ffile-prefix-map=$$($(package)_extract_dir)=/usr
 endef
 
 define $(package)_config_cmds