build libxml2 from source on Linux + link libeconf into msi-interop

Two issues blocked static msi-interop on Alpine:

1. Alpine's libxml2.a is not -fPIC compiled (R_X86_64_PC32 relocations
   against `stderr`/`stdout` globals can not be used when making a
   shared object). Build libxml2 from source on Linux (extending the
   existing macOS branch) so we get a PIC-compiled archive.

2. libblkid.a (transitive dep of glib's static gio) needs econf_*
   symbols from libeconf. Wire libeconf into msi-interop's link_with
   on Linux.

Author: claude

meson.build

@@ -30,8 +30,10 @@ if host_machine.system() == 'windows'
 endif
 
 # --- libxml2 ---
-if host_machine.system() == 'darwin'
-  # Build from source on macOS — Homebrew doesn't provide a static libxml2.a
+# Build from source on macOS (Homebrew doesn't ship static libxml2.a) and
+# Linux (Alpine's libxml2.a isn't compiled with -fPIC, so it can't be embedded
+# into a shared object like libmsi-interop.so).
+if host_machine.system() == 'darwin' or host_machine.system() == 'linux'
   cc = meson.get_compiler('c')
 
   xml2_version_config = configuration_data()
@@ -448,6 +450,9 @@ if host_machine.system() == 'linux'
   # Statically embed all dependencies into libmsi-interop.so so it has no
   # runtime system library dependencies (other than libc) when loaded via
   # P/Invoke from C#. This requires PIC-compiled static libs.
+  # libeconf provides econf_* symbols required by libblkid (transitive dep
+  # of glib's gio static link).
+  msi_interop_link_with += [libeconf]
   msi_interop_link_args += [
     '-Wl,--exclude-libs,ALL',
   ]