Additional include directory given by pkg-config

[bjosv]

The pkg-config installed by either Make or CMake now gives an additional include directory which
enables users to include hiredis headers with:
#include <hiredis/hiredis.h>.
The legacy path is kept for backwards compatibility.

Example before:

# pkg-config hiredis --cflags
-D_FILE_OFFSET_BITS=64 -I/usr/local/include/hiredis

Example output with PR:

# pkg-config hiredis --cflags
-D_FILE_OFFSET_BITS=64 -I/usr/local/include/hiredis -I/usr/local/include

# gcc examples/example.c $(pkg-config --cflags --libs hiredis)  # Still finds <hiredis.h>
                                                                # but also  <hiredis/hiredis.h> if example is updated.

Fixes #1044 and #627

Reference:
See commonly used spec of variable pkgincludedir.