libxml2 @2.9.7_2: ICU include directory not in pkg-config output

[dsedivec]

Since ​libxml2 now builds against ICU, I cannot build software against MacPorts libxml2 without manually including -I$prefix/include when building. Quick example:

$ command -v pkg-config
/opt/local/bin/pkg-config

$ cat repro.c
/* Include inspired by PostGIS's configure script: */
#include <libxml/tree.h>

int main() { return 0; }

$ clang $(pkg-config --cflags libxml-2.0) repro.c $(pkg-config --libs libxml-2.0)
In file included from repro.c:2:
In file included from /opt/local/include/libxml2/libxml/tree.h:1307:
In file included from /opt/local/include/libxml2/libxml/xmlmemory.h:218:
In file included from /opt/local/include/libxml2/libxml/threads.h:35:
In file included from /opt/local/include/libxml2/libxml/globals.h:18:
In file included from /opt/local/include/libxml2/libxml/parser.h:810:
/opt/local/include/libxml2/libxml/encoding.h:31:10: fatal error: 'unicode/ucnv.h' file not found
#include <unicode/ucnv.h>
         ^~~~~~~~~~~~~~~~
1 error generated.

$ clang -I/opt/local/include $(pkg-config --cflags libxml-2.0) repro.c $(pkg-config --libs libxml-2.0)

$ ls a.out
a.out*

After a brief conversation on IRC it was requested that I open a ticket for this, since the ICU cflags/libs flags should probably be included in pkg-config output for libxml-2.0 now. (It was also suggested that maybe this is really an upstream issue, rather than MacPorts.)