Add webos_include_install_paths function

:Release Notes:
Create a header file containing all #defines for WEBOS_INSTALL_* variables and
force it to be included by all C and C++ source files.

:Detailed Notes:
A simple alternative to using `webos_configure_source_files()` to configure
paths in source code is to add a `-DVAR="path"` command line compiler flag
and reference the variable in the source code. For example, instead of:

    fd = open("@WEBOS_INSTALL_SYSCONDIR@/myfile.conf", "r")

script authors often do this in their CMake script

    webos_add_compiler_flag(ALL -DMYCONFPATH="${WEBOS_INSTALL_SYSCONFDIR}")

and this in their source file

    fd = open(MYCONFPATH "/myfile.conf", "r")

which avoids needing to use `webos_configure_source_files()`.

To facilitate this idiom, the `webos_include_install_paths()` function creates
a header file `webospaths.h` under `WEBOS_BINARY_CONFIGURED_DIR` and then adds
the flag `-include WEBOS_BINARY_CONFIGURED_DIR/webos_paths.h` to all compiles.
As a result, all source files in a component using this function gain defined
constants for all system installation paths.

Thus the last line above simply becomes

    fd = open(WEBOS_INSTALL_SYSCONFDIR "/myfile.conf", "r")

:Testing Performed:
The macro was tested locally, and a component then modified to use it and
built successfully.

:QA Notes:

:Issues Addressed:
[GF-23920] Make all WEBOS_INSTALL_* definitions available to source code

Open-webOS-DCO-1.0-Signed-off-by: Keith Derrick <keith.derrick@lge.com>
Change-Id: If21b18ec88e9dcdcbf6060d98928745e2c69c9cd
Reviewed-on: https://g2g.palm.com/3122
Reviewed-by: Build Verification
Reviewed-by: Keith Derrick <keith.derrick@lge.com>
Tested-by: Keith Derrick <keith.derrick@lge.com>