gfortran: Fix has_header implementation with GCC 10
gfortran: Fix has_header implementation with GCC 10

__has_include is not accepted as a pre-processor directive in Fortran
code since GCC 10.

Closes https://github.com/mesonbuild/meson/issues/7017

QuLogic/meson,pexip/meson,becm/meson,pexip/meson,QuLogic/meson,becm/meson,QuLogic/meson,jpakkane/meson,jpakkane/meson,becm/meson,jpakkane/meson,mesonbuild/meson,pexip/meson,jpakkane/meson,mesonbuild/meson,becm/meson,pexip/meson,QuLogic/meson,pexip/meson,pexip/meson,QuLogic/meson,mesonbuild/meson,jpakkane/meson,QuLogic/meson,jpakkane/meson,mesonbuild/meson,becm/meson,pexip/meson,pexip/meson,jpakkane/meson,QuLogic/meson,becm/meson,QuLogic/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,becm/meson,mesonbuild/meson,pexip/meson,becm/meson,mesonbuild/meson,mesonbuild/meson,jpakkane/meson,becm/meson,becm/meson,QuLogic/meson,jpakkane/meson