clean up FC and FFLAGS in Makefiles

[rjleveque]

Application/example Makefiles should say:

FFLAGS ?=

and not mention FC.

Users should be encouraged to set environment variable FFLAG for the flags they generally want. (Add documentation page.)

Makefile.common will check if FC is set to f77 (which it is by default on some systems) and if so, reset to gfortran (since f77 won't work). Users who want to use ifort or another compiler can set environment variable FC.

Default Makefiles will not have -fopenmp

Remove all the commented out lines in various Makefiles, add documentation to list standard combinations of FC and FFLAGS that users might want to use.

This will clean up Makefiles and also avoid developers checking in modified Makefiles with flags that don't work for others.

[mandli]

Related issue in clawpack/clawutil#45