Makefiles: make FC default to gfortran again

[BenjaminDEMAILLE]

Follow-up to #694, noted in #695.

Problem

external_libs/quadpack/Makefile and external_libs/mvtnorm/Makefile both do:

FC   ?= gfortran

That assignment never takes effect. GNU make already defines FC as f77 as a built-in variable, and ?= assigns only when the variable is undefined, which a built-in is not. So a build that does not set FC in the environment uses f77 and stops at:

make: f77: No such file or directory
make: *** [mvt.o] Error 1
make: *** [d1mach.o] Error 1

Machines with a f77 in PATH (often a wrapper around gfortran) do not notice; those without it cannot build regenie unless they know to pass FC=gfortran themselves. It also means the -std=legacy in FLAG is being handed to whatever f77 happens to be, rather than to gfortran as intended.

Fix

Use $(origin FC), which distinguishes make's own default from a value the user supplied:

ifeq ($(origin FC),default)
FC   := gfortran
endif

An FC passed on the command line or exported in the environment still wins, so cross-compilation and non-gfortran toolchains are unaffected.

Testing

macOS 26.6 arm64, GCC 15 gfortran:

$ env -u FC make            # before: make: f77: No such file or directory
gfortran -O2 -DNDEBUG -fPIC -std=legacy -c d1mach.f
...
$ ls libquad.a
libquad.a

$ make FC=gfortran-16       # explicit FC still honoured
gfortran-16 -O2 -DNDEBUG -fPIC -std=legacy -c d1mach.f

Same for external_libs/mvtnorm. Found while packaging regenie for Homebrew, where the build environment does not provide f77.

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