[Vala] Header file not found ?!?

[raum no-log org]

Hello,

I'm trying to output "pcap version" in mingw32 environnement. I've
installed libpcap.vapi, libpcap.deps (from
https://github.com/apmasell/vapis) and all header and lib files from
WpdPack.zip (WinPcap developper kit).

I found in my /opt/include folder :
pcap.h
pcap/pcap.h
...

CPPFLAGS=-I/opt/include
C_LIBRARY_PATH=/usr/include:/mingw/include:/opt/include

simple vala code test_pcap.vala :
---------------------
void main() {
        stdout.printf ("%s\n", PCap.get_version());

}
---------------------

Command to compile test code :
$ valac --pkg gio-2.0 --pkg libpcap  test_pcap.vala
d:/test_pcap.vala.c:8:23: fatal error: pcap/pcap.h: No such file or directory
compilation terminated.
error: cc exited with status 1
Compilation failed: 1 error(s), 0 warning(s)
$

Do you have an advice ? I don't understand why valac didn't find a file
which is obviously present in /opt/include/pcap/ folder ?

Thanks

Raum