Add dummy test checking if linking works properly.

Add FindLAPACKE.cmake and export proxTVconfig.cmake.

FindLAPACKE.cmake is used to handle lapacke installations with no export targets.

Also complete the generation of proxTVconfig.cmake

Add a dummy project to test usage of proxTV from external project.

Add C Interface to Readme

Remove Foo_FOUND in the case FOO is REQUIRED

Update cmake required version to 3.5

This simplifies policy and version set up.

Require a modern lapacke and remove FIND_LAPACKE

This will look for a `lapacke-config.cmake`, that is generated since
lapacke version 3.6. And allow us to remove the buggy FIND_LAPACKE.

Add test for cmake in travis.

Restore FindLAPACKE.cmake

Because travis still uses ubuntu 14.04, with an old lapacke
which does not provide config.cmake.

Fix CMake

Added FindLAPACK
FindLAPACKE CONFIG is not populating LAPACK.

From @jcfr branch: https://github.com/jcfr/proxTV/tree/cmake_support_squashed

Co-authored-by: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>

WIP: Avoid deploy in .travis

Also install cmake with pip (3.12)
Also use find_package(LAPACKE NOCONFIG) for old librart versions

Error:
```
+twine upload --repository-url https://test.pypi.org/legacy/ --username albarji dist/prox_tv-3.3.0-cp27-cp27mu-manylinux1_x86_64.whl
Enter your password:
No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong with the build itself.
Check the details on how to adjust your build configuration on: https://docs.travis-ci.com/user/common-build-problems/#Build-times-out-because-no-output-was-received
```

Only two lapack functions are involved

cmake: Add compile definition depending on LAPACK/EIGEN

option: proxTV_USE_LAPACK (for consistency)
compile definition: PROXTV_USE_LAPACK

All instances of lapack calls have Eigen alternative

Working!

BUG: Add source_dir to include_directories

Fix cmake: remove installing config files in src/CMakeLists

Handled in top CMakelists.txt

Triggered when wrapping ITK

```bash
/usr/bin/ld: _deps/proxtv_fetch-build/src/libitkproxTV-5.0.a(lapackFunctionsWrap.cpp.o):
relocation R_X86_64_PC32 against symbol `_ZTVSt9bad_alloc@@GLIBCXX_3.4' can not be used when making a shared object; recompile with -fPIC
```

Using CXX_CMAKE_FLAGS+="-Wall -Wextra"
Plus Windows specific warnings

COMP: OpenMP_CXX.lib does not exist with Visual Studio

_deps\proxtv_fetch-src\src\TVL1opt_tautstring.cpp(336): warning C4244: '=': conversion from '__int64' to 'int', possible loss of data

Note: LAMBDA_STEPS_TVLP is a a preprocessor definition set to 1.

Windows warnings

This allows building proxTV with Eigen, when Eigen is an internal
target, instead of only allowing find_package(Eigen)

Closes
InsightSoftwareConsortium/ITKTotalVariation#21

with variables:
```cmake
proxTV_INSTALL_INCLUDE_DIR
proxTV_INSTALL_LIB_DIR
proxTV_INSTALL_BIN_DIR
proxTV_INSTALL_CMAKE_DIR
```

These variables where ignored, and other CACHE variables were
used.

`proxTV_INSTALL_CMAKE_DIR` was added with a default value.