The Fortran 95 standard specifies in Part 2 (ISO/IEC 1539-2:2000) varying length character strings. While GNU Fortran currently does not support such strings directly, there exist two Fortran implementations for them, which work with GNU Fortran. They can be found at https://www.fortran.com/iso_varying_string.f95 and at ftp://ftp.nag.co.uk/sc22wg5/ISO_VARYING_STRING/.
Deferred-length character strings of Fortran 2003 supports part of
the features of ISO_VARYING_STRING and should be considered as
replacement. (Namely, allocatable or pointers of the type
character(len=:).)
Part 3 of the Fortran 95 standard (ISO/IEC 1539-3:1998) defines Conditional Compilation, which is not widely used and not directly supported by the GNU Fortran compiler. You can use the program coco to preprocess such files (http://www.daniellnagle.com/coco.html).