Error building curl with c-ares via pkgconfig

[pszemus]

I did this

PKG_CONFIG_PATH=/opt/WP/common.libs/lib/pkgconfig ./configure --enable-ares

I expected the following

curl will build

what happened

configure script exited with error:

checking that c-ares is good and recent enough... no
configure: error: c-ares library defective or too old

config.log:

configure:40149: checking that c-ares is good and recent enough
configure:40176: gcc -o conftest -Werror-implicit-function-declaration -O2 -Wno-system-headers         conftest.c -lcares -lz  >&5
conftest.c:257:18: fatal error: ares.h: No such file or directory
 #include <ares.h>
                  ^
compilation terminated.

ares.h was not found
c-ares is not installed in default install path, because it was built with --prefix but pkgconfig should deliver that custom path, because libcares.pc includes includedir=/opt/WP/common.libs/include:

# cat /opt/WP/common.libs/lib/pkgconfig/libcares.pc 
#***************************************************************************
# Project        ___       __ _ _ __ ___  ___ 
#               / __|____ / _` | '__/ _ \/ __|
#              | (_|_____| (_| | | |  __/\__ \
#               \___|     \__,_|_|  \___||___/
#
prefix=/opt/WP/common.libs
exec_prefix=${prefix}
libdir=${exec_prefix}/lib
includedir=${prefix}/include

Name: c-ares
URL: http://daniel.haxx.se/projects/c-ares/
Description: asynchronous DNS lookup library
Version: -
Requires: 
Requires.private: 
Cflags: -I${includedir} -DCARES_STATICLIB
Libs: -L${libdir} -lcares
Libs.private:

and to confirm that ares.h exists in that path:

# ll /opt/WP/common.libs/include/ares.h
-rw-r--r-- 1 root root 23253 Nov 29 14:01 /opt/WP/common.libs/include/ares.h

It's not a critical issue, because I can build curl with:

CPPFLAGS=-I/opt/WP/common.libs/include LDFLAGS=-L/opt/WP/common.libs/lib ./configure --enable-ares

(config.log then looks like this:)

configure:40149: checking that c-ares is good and recent enough
configure:40176: gcc -o conftest -Werror-implicit-function-declaration -O2 -Wno-system-headers -isystem /opt/WP/common.libs/include    -L/opt/WP/common.libs/lib    conftest.c -lcares -lz  >&5
configure:40176: $? = 0
configure:40178: result: yes

but I'd like to use pkgconfig in the future.

operating system

curl-7.56.1
c-ares-1.13.0
CentOS Linux release 7.4.1708 (Core)

[pszemus]

I've figured out another way of doing this:
PKG_CONFIG="pkg-config --static" PKG_CONFIG_PATH=/opt/WP/common.libs/lib/pkgconfig ./configure --enable-ares
outputs:

checking that c-ares is good and recent enough... yes
(...)
configure: Configured to build curl/libcurl:

  curl version:     7.56.1
  (...)
  resolver:         c-ares

Is it the preferable way?

[bagder]

I could repro and then fixed it with #2658.