Silence the warning: `Non-conforming tab character` in gfortran
Warning: the below answer I originally wrote only applies to gfortran 4.x. The behaviour has been reversed in version 5.x, see the answer by DrOli.
What have you tried so far? Does -Wtabs help? From man gfortran:
-Wtabs
By default, tabs are accepted as whitespace, but tabs are not members of the Fortran Character Set. For continuation lines, a tab followed by a digit between 1 and 9 is supported. -Wno-tabs will cause a warning to be issued if a tab is encountered. Note, -Wno-tabs is active for -pedantic, -std=f95, -std=f2003, -std=f2008 and -Wall.
And -Wall sets -Wno-tabs.
If it doesn't help, it could still be that -Wall overwrites this option. Then you can try manually setting -Wall without the tabs part:
-Wall
Enables commonly used warning options pertaining to usage that we recommend avoiding and that we believe are easy to avoid. This currently includes -Waliasing, -Wampersand, -Wconversion, -Wsurprising, -Wc-binding-type, -Wintrinsics-std, -Wno-tabs, -Wintrinsic-shadow, -Wline-truncation, -Wtarget-lifetime, -Wreal-q-constant and -Wunused.
UPDATE: With GCC/gFortran 5xx (I noticed with 5.3.0), the -Wtabs usage has been "reversed", and as they say, "made more sensible".
See here (https://gcc.gnu.org/gcc-5/changes.html)
Now -Wtabs DOES give the nonconforming warning, whereas -Wno-tabs TURNS OFF the warning (i.e. the opposite of the previous usage).