That's what FreeBSD did also:
https://github.com/freebsd/freebsd-ports/commit/4cab0ea0d675f223f701cdff709d2e65851aa657
The port has more than 50 global symbols defined in more than half a
dozen headers, so let upstream deal with this.
Unsure where to add -fcommon. I saw patches adding it to COPTS, others
added it to CFLAGS or CPPFLAGS. What is preferred?
Index: Makefile
===================================================================
RCS file: /cvs/ports/math/dieharder/Makefile,v
retrieving revision 1.4
diff -u -p -r1.4 Makefile
--- Makefile 12 Jul 2019 20:47:40 -0000 1.4
+++ Makefile 2 Feb 2021 15:20:20 -0000
@@ -3,7 +3,7 @@
COMMENT = random number generator (rng) testing suite
DISTNAME = dieharder-3.31.1
-REVISION = 1
+REVISION = 2
SHARED_LIBS += dieharder 0.0 # 0.0
CATEGORIES = math
@@ -21,6 +21,8 @@ WANTLIB += c gsl gslcblas m
CONFIGURE_STYLE = gnu
CONFIGURE_ENV += CPPFLAGS="-I${LOCALBASE}/include" \
LDFLAGS="-L${LOCALBASE}/lib"
+
+CFLAGS += -fcommon
BUILD_DEPENDS = textproc/latex2html
LIB_DEPENDS = devel/gsl
