ICU headers not found when building libxml

[joaocandre]

I'm trying to cross-build eilmeldung for arm64v8 (aarch64-unknown-linux-gnu), but I'm encountering an issue with the required dependencies, particularly libicu-dev:

--- stderr
  warning: ISO C99 requires whitespace after the macro name [-Wc99-extensions]
  /usr/include/libxml2/libxml/xmlversion.h:124:9: warning: 'LIBXML_OUTPUT_ENABLED' macro redefined [-Wmacro-redefined]
  note: previous definition is here
  /usr/include/libxml2/libxml/xmlversion.h:223:9: warning: 'LIBXML_C14N_ENABLED' macro redefined [-Wmacro-redefined]
  note: previous definition is here
  /usr/include/libxml2/libxml/encoding.h:31:10: fatal error: 'unicode/ucnv.h' file not found

I'm installing the required dependencies during prebuild, as per Cross.toml:

[target.aarch64-unknown-linux-gnu]
pre-build = [
    #"dpkg --add-architecture arm64",
    "apt-get update",
    "apt-get install -y libssl-dev libxml2-dev libsqlite3-dev libicu-dev clang"
]

By default libicu "unicode" headers get installed into /usr/include/x86_64-linux-gnu (and are already part of the default toolchain, so installing is redundant anyway); alternatively, if I explicitly install the arm64 package they are placed into /usr/include/aarch64-linux-gnu, they don't seem to be part of the include path.

What am I missing in the configuration?