Makefile.kokkos gets gcc-toolchain wrong if gcc is cached

[junghans]

When ccache is installed, g++ and clang++ are a symlinks to ccache in /usr/lib/ccahe and hence KOKKOS_INTERNAL_GCC_PATH and later the --gcc-toolchain option are wrong leading to an error like:

/usr/local/clang-5.0.0/bin/clang++  -I./ -I/home/travis/build/junghans/kokkos/core/src -I/home/travis/build/junghans/kokkos/containers/src -I/home/travis/build/junghans/kokkos/algorithms/src -I/home/travis/build/junghans/kokkos/core/src/eti -Wall -Wshadow -pedantic -Wsign-compare -Wtype-limits -Wuninitialized --std=c++11 --gcc-toolchain=/usr/lib/ccache/g++ -O3 -c /home/travis/build/junghans/kokkos/core/src/impl/Kokkos_Core.cpp
In file included from /home/travis/build/junghans/kokkos/core/src/impl/Kokkos_Core.cpp:44:
In file included from /home/travis/build/junghans/kokkos/core/src/Kokkos_Core.hpp:50:
In file included from /home/travis/build/junghans/kokkos/core/src/Kokkos_Core_fwd.hpp:52:
/home/travis/build/junghans/kokkos/core/src/impl/Kokkos_Utilities.hpp:48:10: fatal error: 'cstdint' file not found
#include <cstdint>
         ^~~~~~~~~
1 error generated.

when compiling with clang.

The is no option to overwrite KOKKOS_INTERNAL_GCC_PATH with generate_makefile.bash, but as a workaround one can do:

make KOKKOS_INTERNAL_GCC_TOOLCHAIN=/usr
make install KOKKOS_INTERNAL_GCC_TOOLCHAIN=/usr

which is what we had to do for Cabana in ECP-copa/Cabana@e6029bb.

An easy way to reproduce this issue is to enable ccache in travis, i.e.
junghans@5f65b86
which leads to a build failure:
https://travis-ci.org/junghans/kokkos/builds/438615932 for clang.

An option to overwrite the gcc-toolchain option at the generate_makefile.bash level would be nice.