Remove duplicate DCUSTOM_DATA_INSTALL_PATH

Within 5 lines CUSTOM_DATA_INSTALL_PATH is defined twice resulting in lots of
compiler warnings; eg

[ 11%] Building CXX object source/shared_lib/sources/streflop/CMakeFiles/streflop.dir/libm/flt-32/s_atanf.cpp.o
In file included from <built-in>:175:
<command line>:7:9: warning: 'CUSTOM_DATA_INSTALL_PATH' macro redefined
        ^
<command line>:2:9: note: previous definition is here
        ^

This removes the second definition.