diff --git a/external_libs/mvtnorm/Makefile b/external_libs/mvtnorm/Makefile
index 29f883db..ad0b1090 100644
--- a/external_libs/mvtnorm/Makefile
+++ b/external_libs/mvtnorm/Makefile
@@ -2,7 +2,10 @@ FLAG  =-O2 -DNDEBUG -fPIC
 AR   ?= ar
 CXX  ?= g++
 CC   ?= gcc
-FC   ?= gfortran
+# make defines FC as `f77` itself, so `?=` would never take effect here.
+ifeq ($(origin FC),default)
+FC   := gfortran
+endif
 
 UNAME_S := $(shell uname -s)
 UNAME_M := $(shell uname -m)
diff --git a/external_libs/quadpack/Makefile b/external_libs/quadpack/Makefile
index f70f0b1f..9971d0e7 100644
--- a/external_libs/quadpack/Makefile
+++ b/external_libs/quadpack/Makefile
@@ -1,6 +1,9 @@
 FLAG=-O2 -DNDEBUG -fPIC -std=legacy
 AR   ?= ar
-FC   ?= gfortran
+# make defines FC as `f77` itself, so `?=` would never take effect here.
+ifeq ($(origin FC),default)
+FC   := gfortran
+endif
 
 UNAME_S := $(shell uname -s)
 UNAME_M := $(shell uname -m)