includedir in libcurl.pc(for pkg-config) may be wrong

Environment
=============
===
# cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=14.04
DISTRIB_CODENAME=trusty
DISTRIB_DESCRIPTION="Ubuntu 14.04.1 LTS"
===

Issue
============

I installed libcurl4-openssl-dev package as follows.

===
# apt-get update
# apt-get install -y libcurl4-openssl-dev
===

After that, I try to compile some program depending on libcurl with pkg-config, but header file of curl can't be found.

===
# cat curl_test.c
#include "curl.h"

int main(args)
{
  return 0;
}

# gcc `pkg-config libcurl --cflags --libs` curl_test.c
curl_test.c:1:18: fatal error: curl.h: No such file or directory
 #include "curl.h"
                  ^
compilation terminated.
===

I've checked pc file of libcurl, and includedir(=/usr/include) is different from the path curl.h exists.

===
# cat /usr/lib/x86_64-linux-gnu/pkgconfig/libcurl.pc
...
prefix=/usr
...
includedir=${prefix}/include # = /usr/include
...
Cflags: -I${includedir}
...

# ls /usr/include/ | grep curl.h
# ls /usr/include/curl/ | grep curl.h
curl.h
===