Provide a way to specify where libgs is

[ryandesign]

You look for libgs in a variety of locations varying depending on the OS:

GLE/src/gui/qgslibloader.cpp

Lines 295 to 310 in 97cf2ca

	#ifdef Q_OS_HURD
	tryLocationLoop("/usr/lib/i386-gnu/");
	#endif // HURD
	#ifdef Q_OS_LINUX
	#if defined(__x86_64__) || defined(__ppc64__) || defined (__s390x__) || defined (__sparc64__)
	// try 64 bit libraries on 64 bit system
	tryLocationLoop("/usr/lib64");
	tryLocationLoop("/usr/local/lib64");
	#endif // 64 bit
	tryLocationLoop("/usr/lib");
	tryLocationLoop("/usr/local/lib");
	#endif // Q_OS_LINUX
	#ifdef Q_OS_MACOS
	tryLocation("/usr/lib/libgs.dylib");
	tryLocation("/usr/local/lib/libgs.dylib");
	#endif

but on any OS those locations are not necessarily where libgs is actually located. Please provide a CMake option to specify where libgs is actually located and then use that at runtime.

[ryandesign]

In fact I bet there's a CMake find_package incantation that could locate libgs for you or could be overridden by the user in the usual way if desired; then you just tell qgslibloader.cpp to load libgs from that location.

[vlabella]

Option EXTRA_GSLIB_SEARCH_LOCATION has been added to the cmake file to specify extra search location in src/gui/qgslibloader.cpp. it needs testing.