btrfs-progs: build: static should pick up EXTRA_LDFLAGS and SUBST_LDFLAGS

Concrete use-case:

- I want a static build to run in a small test VM.
- RedHat regrettably does not provide util-linux static libraries
- like libuuid.a and libblkid.a.
- So, one has to build them from source.
- Installing these to /usr is lame.
- This change lets me do make btrfs.static EXTRA_LDFLAGS=-L/path/to/util-linux-2.38/.libs

Pull-request: #563
Author: Alexey <snarkmaster@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Makefile

@@ -134,7 +134,7 @@ LIBBTRFS_LIBS = $(LIBS_BASE) $(LIBS_CRYPTO)
 
 # Static compilation flags
 STATIC_CFLAGS = $(CFLAGS) -ffunction-sections -fdata-sections -DSTATIC_BUILD
-STATIC_LDFLAGS = -static -Wl,--gc-sections
+STATIC_LDFLAGS = $(SUBST_LDFLAGS) $(EXTRA_LDFLAGS) -static -Wl,--gc-sections
 STATIC_LIBS = $(STATIC_LIBS_BASE)
 
 # don't use FORTIFY with sparse because glibc with FORTIFY can