Support finding libquadmath with native compiler support

[masterleinad]

gcc oftentimes bundles libquadmath and doesn't need any extra libraries or include directories (which would not be contained in CMake's search paths anyway).
Hence, try if the compiler supports libquadmath natively first before doing the standard package search.

KOKKOS_IMPORT_TPL needed a workaround to be able to create a consistent target whether KOKKOS_QUADMATH_COMPILER_SUPPORT is true or not.

This should in particular help with testing libquadmath support.

[dalg24]

Please apply my suggestion
Looks good other than that

[PhilMiller]

This seems like a good candidate for 3.7

Unrelated changes pushed post approval

[masterleinad]

Unrelated changes pushed post approval

They are not unrelated but make sure the detection also works with -Werror and without extra include path.

[masterleinad]

This seems like a good candidate for 3.7

I think this is not a bug fix so I would abstain from pulling more stuff in.

[PhilMiller]

I was looking at it as something that would ease our support burden from users who have trouble with this aspect of the build/installation process

[masterleinad]

There are a bunch of CI failures (mostly timing based but OpenMPTarget can't find LIBDL???) but this pull request only affects the gcc-5.3.1 build which works with

-- Performing Test KOKKOS_QUADMATH_COMPILER_SUPPORT
-- Performing Test KOKKOS_QUADMATH_COMPILER_SUPPORT - Success

[dalg24]

Why is this needed? Make sure this is in the description of the PR

[masterleinad]

I added a comment. Without this, we are getting

-- Performing Test KOKKOS_QUADMATH_COMPILER_SUPPORT
-- Performing Test KOKKOS_QUADMATH_COMPILER_SUPPORT - Success
CMake Error at cmake/kokkos_functions.cmake:187 (INSTALL):
  INSTALL TARGETS given target "Kokkos::LIBQUADMATH" which is an alias.
Call Stack (most recent call first):
  cmake/kokkos_functions.cmake:288 (KOKKOS_EXPORT_IMPORTED_TPL)
  cmake/kokkos_tpls.cmake:86 (KOKKOS_IMPORT_TPL)
  cmake/kokkos_tribits.cmake:247 (INCLUDE)
  CMakeLists.txt:188 (KOKKOS_SETUP_BUILD_ENVIRONMENT)