Move GNUInstallDirs include

[EmbersArc]

I noticed an issue with the CMake installation: The line to import the include directory was in the wrong place which lead to the important preprocessor definitions LDL_LONG and DLONG not being included correctly . I tested this change on Linux.

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This change is 

[coveralls]

Coverage remained the same at 0.0% when pulling baf3af6 on EmbersArc:cmake_fix into 3d2f48b on embotech:develop.

[csorvagep]

I noticed this as well, but because this resulted to an invalid include directory of /usr/local//ecos instead of /usr/local/include/ecos.

I did the same modification, although I'm not sure, of the right location. Maybe this fits better between the set() directives and the add_library(). But I'm only splitting hairs, please accept this PR.