Merge bitcoin#28846: depends: fix libmultiprocess build on aarch64

bde8d63 depends: build libmultiprocess with position independant code (fanquake)
506634d depends: always install libmultiprocess to /lib (fanquake)
beb3096 depends: always install capnp to /lib (fanquake)

Pull request description:

  Change to always install libmultiprocess into `lib/`. On some systems (my Fedora aarch64 box), libmultiprocess was being installed into `lib64/`, and then configure would fail to pick it up, because we only add `lib/` to pkgconfig/ldflags out of depends. Rather than adding lib64 to those, I opted for installing libmultiprocess into lib, with every other dependency we build.

  This was broken in our build after bitcoin-core/libmultiprocess#79 upstream.

ACKs for top commit:
  ryanofsky:
    Code review ACK bde8d63. Only changes since last review were reverting the native_capnp change as suggested, and changing the order of the first two commits.

Tree-SHA512: ddd547e4ac224f2f199c569efd91104db7f2c243b124f9535aa0d9377315775ac566d699101580ce45ddd6676ad3e0c8cbe256334eeed9548205c2fa04d02102

Author: fanquake

depends/packages/capnp.mk

@@ -5,10 +5,15 @@ $(package)_download_file=$(native_$(package)_download_file)
 $(package)_file_name=$(native_$(package)_file_name)
 $(package)_sha256_hash=$(native_$(package)_sha256_hash)
 
+# Hardcode library install path to "lib" to match the PKG_CONFIG_PATH
+# setting in depends/config.site.in, which also hardcodes "lib".
+# Without this setting, cmake by default would use the OS library
+# directory, which might be "lib64" or something else, not "lib", on multiarch systems.
 define $(package)_set_vars :=
-$(package)_config_opts := -DBUILD_TESTING=OFF
-$(package)_config_opts += -DWITH_OPENSSL=OFF
-$(package)_config_opts += -DWITH_ZLIB=OFF
+  $(package)_config_opts := -DBUILD_TESTING=OFF
+  $(package)_config_opts += -DWITH_OPENSSL=OFF
+  $(package)_config_opts += -DWITH_ZLIB=OFF
+  $(package)_config_opts += -DCMAKE_INSTALL_LIBDIR=lib/
 endef
 
 define $(package)_config_cmds

depends/packages/libmultiprocess.mk

@@ -8,7 +8,13 @@ ifneq ($(host),$(build))
 $(package)_dependencies += native_capnp
 endif
 
+# Hardcode library install path to "lib" to match the PKG_CONFIG_PATH
+# setting in depends/config.site.in, which also hardcodes "lib".
+# Without this setting, cmake by default would use the OS library
+# directory, which might be "lib64" or something else, not "lib", on multiarch systems.
 define $(package)_set_vars :=
+$(package)_config_opts += -DCMAKE_INSTALL_LIBDIR=lib/
+$(package)_config_opts += -DCMAKE_POSITION_INDEPENDENT_CODE=ON
 ifneq ($(host),$(build))
 $(package)_config_opts := -DCAPNP_EXECUTABLE="$$(native_capnp_prefixbin)/capnp"
 $(package)_config_opts += -DCAPNPC_CXX_EXECUTABLE="$$(native_capnp_prefixbin)/capnpc-c++"