Patch taken from nix.

commit fdda59736ddc048cf38a2c7103f4f5d9eeaf995e
Author: Ben Gamari <ben@smart-cactus.org>
Date:   Tue Oct 17 10:51:34 2017 -0400

    Try implementing cross-compilation

diff --git a/bc/Makefile.am b/bc/Makefile.am
index d9d412e..fdef633 100644
--- a/bc/Makefile.am
+++ b/bc/Makefile.am
@@ -17,6 +17,7 @@ MAINTAINERCLEANFILES = Makefile.in bc.c bc.h scan.c \
 
 AM_CPPFLAGS = -I$(srcdir) -I$(srcdir)/../h
 LIBBC = ../lib/libbc.a
+LIBBC_HOST = ../lib/libbc_host.a
 LIBL = @LEXLIB@
 LDADD = $(LIBBC) $(LIBL) @READLINELIB@
 
@@ -29,12 +30,20 @@ $(PROGRAMS): libmath.h $(LIBBC)
 scan.o: bc.h
 global.o: libmath.h
+
+main_host.c : main.c
+	cp $< $@
 
-fbcOBJ = main.o bc.o scan.o execute.o load.o storage.o util.o warranty.o
+fbcOBJ = $(addsuffix _host,main.o bc.o scan.o execute.o load.o storage.o util.o warranty.o)
+
+%.o_host : CC:=$(CC_FOR_BUILD)
+
+%.o_host : %.c
+	$(COMPILE) -c $(CFLAGS) $(INCLUDES) -o $@ $<
 
-libmath.h: libmath.b $(fbcOBJ) $(LIBBC)
+libmath.h: libmath.b $(fbcOBJ) $(LIBBC_HOST)
 	echo '{0}' > libmath.h
-	$(MAKE) global.o
-	$(LINK) -o fbc $(fbcOBJ) global.o $(LIBBC) $(LIBL) $(READLINELIB) $(LIBS)
+	$(MAKE) global.o_host
+	$(CC_FOR_BUILD) -o fbc $(fbcOBJ) global.o_host $(LIBBC_HOST) $(LIBL) ${READLINELIB} $(LIBS)
 	./fbc -c $(srcdir)/libmath.b </dev/null >libmath.h
 	$(srcdir)/fix-libmath_h
 	rm -f ./fbc ./global.o
diff --git a/configure.ac b/configure.ac
index fc74573..5cabb73 100644
--- a/configure.ac
+++ b/configure.ac
@@ -20,6 +20,7 @@ m4_define([dc_version], 1.4.1)
 
 AC_INIT([bc],[bc_version])
 AC_CONFIG_SRCDIR(doc/bc.1)
+AC_CONFIG_MACRO_DIR([m4])
 AM_INIT_AUTOMAKE([dist-bzip2])
 AC_CONFIG_HEADERS(config.h)
 
@@ -35,6 +36,7 @@ AC_DEFINE([DC_COPYRIGHT],
 	  [Define the dc copyright line.])
 
 AC_PROG_CC
+AX_CC_FOR_BUILD
 AC_USE_SYSTEM_EXTENSIONS
 
 AM_PROG_LEX
diff --git a/lib/Makefile.am b/lib/Makefile.am
index ec4bf59..c670f5b 100644
--- a/lib/Makefile.am
+++ b/lib/Makefile.am
@@ -1,5 +1,5 @@
 ## Process this file with automake to produce Makefile.in
-noinst_LIBRARIES = libbc.a
+noinst_LIBRARIES = libbc.a libbc_host.a
 
 AM_CPPFLAGS = -I. -I.. -I$(srcdir)/../h
 
@@ -24,3 +24,11 @@ testmul: testmul.o number.o
 
 specialnumber: newnumber.o
 	cp newnumber.o number.o
+
+%.o_host : CC:=$(CC_FOR_BUILD)
+%.o_host : %.c
+	$(COMPILE) -c $(CFLAGS) $(INCLUDES) -o $@ $<
+
+libbc_host.a : $(addsuffix _host,$(libbc_a_OBJECTS))
+	ar cru $@ $+
+	ranlib $@
diff --git a/m4/cc_for_build.m4 b/m4/cc_for_build.m4
new file mode 100644
index 0000000..c62ffad
--- /dev/null
+++ b/m4/cc_for_build.m4
@@ -0,0 +1,77 @@
+# ===========================================================================
+#     https://www.gnu.org/software/autoconf-archive/ax_cc_for_build.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_CC_FOR_BUILD
+#
+# DESCRIPTION
+#
+#   Find a build-time compiler. Sets CC_FOR_BUILD and EXEEXT_FOR_BUILD.
+#
+# LICENSE
+#
+#   Copyright (c) 2010 Reuben Thomas <rrt@sc3d.org>
+#   Copyright (c) 1999 Richard Henderson <rth@redhat.com>
+#
+#   This program is free software: you can redistribute it and/or modify it
+#   under the terms of the GNU General Public License as published by the
+#   Free Software Foundation, either version 3 of the License, or (at your
+#   option) any later version.
+#
+#   This program is distributed in the hope that it will be useful, but
+#   WITHOUT ANY WARRANTY; without even the implied warranty of
+#   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
+#   Public License for more details.
+#
+#   You should have received a copy of the GNU General Public License along
+#   with this program. If not, see <https://www.gnu.org/licenses/>.
+#
+#   As a special exception, the respective Autoconf Macro's copyright owner
+#   gives unlimited permission to copy, distribute and modify the configure
+#   scripts that are the output of Autoconf when processing the Macro. You
+#   need not follow the terms of the GNU General Public License when using
+#   or distributing such scripts, even though portions of the text of the
+#   Macro appear in them. The GNU General Public License (GPL) does govern
+#   all other use of the material that constitutes the Autoconf Macro.
+#
+#   This special exception to the GPL applies to versions of the Autoconf
+#   Macro released by the Autoconf Archive. When you make and distribute a
+#   modified version of the Autoconf Macro, you may extend this special
+#   exception to the GPL to apply to your modified version as well.
+
+#serial 3
+
+dnl Get a default for CC_FOR_BUILD to put into Makefile.
+AC_DEFUN([AX_CC_FOR_BUILD],
+[# Put a plausible default for CC_FOR_BUILD in Makefile.
+if test -z "$CC_FOR_BUILD"; then
+  if test "x$cross_compiling" = "xno"; then
+    CC_FOR_BUILD='$(CC)'
+  else
+    CC_FOR_BUILD=gcc
+  fi
+fi
+AC_SUBST(CC_FOR_BUILD)
+# Also set EXEEXT_FOR_BUILD.
+if test "x$cross_compiling" = "xno"; then
+  EXEEXT_FOR_BUILD='$(EXEEXT)'
+else
+  AC_CACHE_CHECK([for build system executable suffix], bfd_cv_build_exeext,
+    [rm -f conftest*
+     echo 'int main () { return 0; }' > conftest.c
+     bfd_cv_build_exeext=
+     ${CC_FOR_BUILD} -o conftest conftest.c 1>&5 2>&5
+     for file in conftest.*; do
+       case $file in
+       *.c | *.o | *.obj | *.ilk | *.pdb) ;;
+       *) bfd_cv_build_exeext=`echo $file | sed -e s/conftest//` ;;
+       esac
+     done
+     rm -f conftest*
+     test x"${bfd_cv_build_exeext}" = x && bfd_cv_build_exeext=no])
+  EXEEXT_FOR_BUILD=""
+  test x"${bfd_cv_build_exeext}" != xno && EXEEXT_FOR_BUILD=${bfd_cv_build_exeext}
+fi
+AC_SUBST(EXEEXT_FOR_BUILD)])dnl