Allow for CMake to use absolute install paths

[jonringer]

Certain build systems such as Nix will install dev related items separately from library and runtime outputs. The current:

# libcares.pc.cmake
prefix=@CMAKE_INSTALL_PREFIX@
includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@

will create paths such as includedir=/nix/store/pgvk94m98y2sq5x91mpz2nsm7pd2yqkp-c-ares-1.18.1-dev//nix/store/pgvk94m98y2sq5x91mpz2nsm7pd2yqkp-c-ares-1.18.1-dev/include which should be includedir=/nix/store/pgvk94m98y2sq5x91mpz2nsm7pd2yqkp-c-ares-1.18.1-dev/include

Result of PR:

cat ./result-dev/lib/pkgconfig/libcares.pc
#***************************************************************************
# Project        ___       __ _ _ __ ___  ___
#               / __|____ / _` | '__/ _ \/ __|
#              | (_|_____| (_| | | |  __/\__ \
#               \___|     \__,_|_|  \___||___/
#
exec_prefix=/nix/store/shskwbrmq0nl2xvwc53xfby6x7xymils-c-ares-1.18.1/bin
libdir=/nix/store/shskwbrmq0nl2xvwc53xfby6x7xymils-c-ares-1.18.1/lib
includedir=/nix/store/i14bqixhc0gi30ajlgbn31g0p77asj5n-c-ares-1.18.1-dev/include

Name: c-ares
URL: https://c-ares.org/
Description: asynchronous DNS lookup library
Version: 1.18.1
Requires:
Requires.private:
Cflags: -I${includedir}
Libs: -L${libdir} -lcares
Libs.private:

[jonringer]

failing CI gate looks unrelated, the autotool/configure step is failing:

configure: error: in `/tmp/cirrus-ci-build/atoolsbld':
configure: error: C compiler cannot create executables
See `config.log' for more details
scan-build: Removing directory '/tmp/scan-build-2022-10-13-200111-18915-1' because it contains no reports.
scan-build: No bugs found.