With the former commit that fixes the wrongfully
exported VegaFEM, VegaFEM was not found anymore
by iMSTK. This is because its Config.cmake file
was not configured in VegaFEM_DIR aka the top-level
of VegaFEM build directory.

This commit:
- correctly places the config & target files
- removes the exports(TARGETS ...) for Vega modules
- corrects the finding of the pthreads package if needed

export(PACKAGE VegaFEM) was resulting in Vega being registered
in a CMake Package Registry :
https://cmake.org/Wiki/CMake/Tutorials/Package_Registry

Because of this, an incorrect version of VEGA could be directly
linked against when doing find_package(VegaFEM).

This should only be done when creating a package for installation:
https://cmake.org/cmake/help/git-master/manual/cmake-packages.7.html#creating-packages

Use __linux__

1) Configuring FindPThreads.cmake to find this
file when doing a find package of VegaFEM, since
its config file is in the build directory

2) Do find_package(PThreads) in the VegaFEMConfig
file.

WARNING: Should need some attention to work without
PThreads, for Unix for example.

Also no need to do the find packages in the
config file: if this file exists and is found
it means that VegaFEM dependencies are built
and found since those occur earlier in the
top level CMakeLists.txt

We'll handle this from the superbuild instead.

Update configuration file so it exports more of vega variables.

Fix warnings related to conversion to string literals to char*

Re-factor the linear solver and sparse integrator

Make solver interfaces more generic makes it easier to plug external linear solver in to the time stepping method.

Conflicts:
	src/util/interactiveDeformableSimulator/interactiveDeformableSimulator.cpp

Some changes to get things building on OS X again.

... when threads and OpenGL support are off.

It looks for the Xmu library even though it is
not a dependency of freeglut3.

... to see if it builds without BLAS/LAPACK.