CMAKE_INSTALL_PREFIX was incorrectly being prepended to the install d… #461

Job	Run time
build-drivers-release	4m 49s
build-postlib-release	9m 10s
build-all-debug	4m 37s
build-all-debug-single	3m 32s
build-test-uadriver-debug	2m 54s
rtest-modules-debug	4m 32s
rtest-module-drivers	2m 11s
build-interfaces-release	3m 21s
build-fastfarm-release	10m 32s
build-openfast-release	3m 19s
rtest-OF	6m 1s
rtest-OF-5MW_OC3Mnpl_DLL_WTurb_WavesIrr	3m 7s
rtest-OF-5MW_OC4Semi_WSt_WavesWN	1m 24s
rtest-OF-5MW_OC4Jckt_DLL_WTurb_WavesIrr_MGrowth	1m 11s
rtest-OF-5MW_Land_BD_DLL_WTurb	2m 10s
rtest-OF-linearization	3m 45s
rtest-OF-5MW_OC3Trpd_DLL_WSt_WavesReg	4m 37s
rtest-interfaces	3m 27s
rtest-FF	2m 3s
	1h 16m 42s

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