Make cross-building work.

hddemux fails to cross build from source, because it hard codes build
architecture build tools. The attached patch makes the relevant tools
substitutable and hddemux cross buildable. Please consider applying it.

See https://bugs.debian.org/928996

Makefile

@@ -2,10 +2,11 @@
 
 CFLAGS += -D_GNU_SOURCE -g -O3
 
-CFLAGS += $(shell pkg-config --cflags libuv)
-CFLAGS += $(shell pkg-config --cflags libsystemd)
-LDFLAGS += $(shell pkg-config --libs libuv)
-LDFLAGS += $(shell pkg-config --libs libsystemd)
+PKG_CONFIG ?= pkg-config
+CFLAGS += $(shell $(PKG_CONFIG) --cflags libuv)
+CFLAGS += $(shell $(PKG_CONFIG) --cflags libsystemd)
+LDFLAGS += $(shell $(PKG_CONFIG) --libs libuv)
+LDFLAGS += $(shell $(PKG_CONFIG) --libs libsystemd)
 
 all: hddemux hddemux.1
 
@@ -13,7 +14,7 @@ check: hddemux
 	PATH=.:$$PATH ./testsuite
 
 hddemux: hddemux.c
-	gcc $(CPPFLAGS) $(CFLAGS) $< -Wl,--as-needed $(LDFLAGS) -std=c11 -pedantic -Wall -Werror -o $@
+	$(CC) $(CPPFLAGS) $(CFLAGS) $< -Wl,--as-needed $(LDFLAGS) -std=c11 -pedantic -Wall -Werror -o $@
 
 hddemux.1: hddemux.1.md
 	pandoc -s -f markdown -t man -o $@ $<