contrib: fix pkg-config tool detection

Attempt to set the platform-specific pkg-config tool *before* the
native/generic one.

contrib/src/main.mak

@@ -235,7 +235,6 @@ export ACLOCAL_AMFLAGS
 # Tools #
 #########
 
-PKG_CONFIG ?= pkg-config
 need_pkg = $(shell $(PKG_CONFIG) $(1) || echo 1)
 
 ifdef HAVE_CROSS_COMPILE
@@ -250,6 +249,8 @@ need_pkg = $(shell PKG_CONFIG_LIBDIR=$(PKG_CONFIG_LIBDIR) $(PKG_CONFIG) $(1) ||
 endif
 endif # HAVE_CROSS_COMPILE
 
+PKG_CONFIG ?= pkg-config
+
 PKG_CONFIG_PATH := $(PREFIX)/lib/pkgconfig:$(PKG_CONFIG_PATH)
 ifeq ($(findstring mingw32,$(BUILD)),mingw32)
 PKG_CONFIG_PATH := $(shell cygpath -pm ${PKG_CONFIG_PATH})