bug#34946: [bug-gnu-emacs] proposed configure.ac change for emacs-26.2

[Nelson H. F. Beebe]

Testing of the release candidate for emacs-26.2 on Oracle Solaris 11.4
shows that a build fails because lcms2.h is not found in /usr/include.
On that system, and on Hipster, an OpenSolaris relative, that file is
present in /usr/include/lcms.

Discussions with Eli Zaretskii <eliz@gnu.org> suggest that emacs'
configure.ac needs to use pkg-config to identify the correct include
directory for lcms2.h:

        % pkg-config --cflags lcms2
        -I/usr/include/lcms

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