qRestAPIConfig - Do not explicitly find library - Rely on exported target

Since target are exported, it is not needed to use "find_library" to
get the full path of the library. After calling "find_package(qRestAPI)",
project can safely use "${qRestAPI_LIBRARIES}" or "qRestAPI" within the
"target_link_libraries" call.

Not also that since the library is imported
as a CMake target, using "target_link_directories" is also obsolete.
Support windows static build

Following commit 5c056f2, the project could be built as a shared library
this caused problem on windows where the qRestAPI_EXPORT macro was
also defined in the case of static build causing an "inconsistent linkage"
error.

This commit explicitly define qRestAPI_EXPORT based on the
BUILD_SHARED_LIBS option.
Merge branch 'support-windows-static-build'

* support-windows-static-build:
  Support windows static build
Ensure qRestAPI_Export.h is available for project depending on qRestAPI

Following commit 6339e0, "qRestAPI_Export.h.in" is configured into
"qRestAPI_Export.h" which is located in the build tree.

This commit ensure that the folder containing the configured file
is added to the list of directory to be included by project calling
"find_package(qRestAPI [...])"
Define qRestAPI_EXPORT on Unix platform

While commit 6339e0cc fixed windows static build it also broke unix one.
This commit fixes the problem.
Merge branch 'support-windows-static-build'

* support-windows-static-build:
  Define qRestAPI_EXPORT on Unix platform
  Ensure qRestAPI_Export.h is available for project depending on qRestAPI
Fix build CLang build error by making static helper method public.

For the record, the reason this fails on Mac and not on Linux is
the following :
http://clang-developers.42468.n3.nabble.com/Access-to-static-protected-member-from-friends-of-derived-classes-td4030444.html

The Q_DECLARE_PRIVATE macro declares the qXnatAPIPrivate class and makes
it a friend of qXnatAPI, which is sufficient for G++ to give it access to
qRestAPI's static protected methods, but the Clang developers have a
different understanding of the standard, so it doesn't work on Clang.

Thanks to Steve Pieper, Florian Vichot and Sascha Zelzer for their
help fixing the issue.

See #334
Remove definition of method "qVariantMapToString" not implemented or used.

This method has originally been added by commit 7d2755

The method "qVariantMapListToString" (added in a074efc) already fulfill
the role intended by "qVariantMapToString"