cmake: Avoid overwriting PKG_CONFIG_PATH env var

cmake: Avoid overwriting PKG_CONFIG_PATH env var

The CMake config file installed by DBus will run in the context of other
projects. Consequently, changing the value of the PKG_CONFIG_DIR,
PKG_CONFIG_PATH or PKG_CONFIG_LIBDIR environment variables will affect
any further calls to pkg-config made by such projects, which can cause
problems.

A common case of this happening are pkg-config files installed in
usr/share/pkgconfig for .pc files that are architecture-independent, as
for example systemd does.

Avoid clobbering the environment variables by saving and restoring their
values. Note that for some of the variables, setting them to an empty
string is different from not setting them at all.

Signed-off-by: Clemens Lang <clemens.lang@bmw-carit.de>
(cherry picked from commit 3525cc045d4c683dfc6048f5be795cc372c323a3)
Closes: dbus#267