iguana: add environment variables for ROOT and clas12root support

[c-dilks]

Supports:

• feat: add ROOT macro example iguana#210
• Iguana clas12root#69

Variable	Modification
ROOT_INCLUDE_PATH	Iguana header files, for usage in ROOT
IGUANA	the path to the Iguana installation prefix, equivalent to pkg-config iguana --variable prefix; this is only for consumers that do not use pkg-config or the other standard environment variables, however usage of this variable is discouraged since the installation layout may vary

• also removes the Fortran-specific environment variables, which are currently only used on a testing branch

[c-dilks]

@baltzell we may also need fmt and yaml-cpp in ROOT_INCLUDE_PATH