Add generation of pkg-config file

[Shelnutt2]

Add generation of pkg-config file for installation.

[eddelbuettel]

Lovely:

[....earlier stuff omitted...]
-- Installing: /usr/local/lib/cmake/TileDB/TileDBTargets-debug.cmake
-- Installing: /usr/local/lib/pkgconfig/tiledb.pc
make: Leaving directory '/home/edd/git/tiledb/build/tiledb'
edd@rob:~/git/tiledb/build(sethshelnutt/ch2363/add-pkg-config-support-for-main-tiledb-build)$ pkg-config --list-all | grep tiledb
tiledb                         TileDB - The Storage Engine for Data Science
edd@rob:~/git/tiledb/build(sethshelnutt/ch2363/add-pkg-config-support-for-main-tiledb-build)$ pkg-config --libs tiledb
-L/usr/local/lib -ltiledb
edd@rob:~/git/tiledb/build(sethshelnutt/ch2363/add-pkg-config-support-for-main-tiledb-build)$ pkg-config --cflags tiledb
-I/usr/local/include/tiledb
edd@rob:~/git/tiledb/build(sethshelnutt/ch2363/add-pkg-config-support-for-main-tiledb-build)$