contrib: download gmp from gmplib.org

Change-Id: I15a5f7729a5296ecc6e7e1ad0b3ec1ae10797269

contrib/src/gmp/SHA512SUMS

-3b684c9bcb9ede2b7e54d0ba4c9764bfa17c20d4f3000017c553b6f1e135b536949580ff37341680c25dc236cfe0ba1db8cfdfe619ce013656189ef0871b89f8  gmp-6.3.0.tar.bz2
+e85a0dab5195889948a3462189f0e0598d331d3457612e2d3350799dba2e244316d256f8161df5219538eb003e4b5343f989aaa00f96321559063ed8c8f29fd2  gmp-6.3.0.tar.xz

contrib/src/gmp/rules.mak

@@ -3,18 +3,18 @@
 
 GMP_VERSION := 6.3.0
 PKG_CPE += cpe:2.3:a:gmplib:gmp:$(GMP_VERSION):*:*:*:*:*:*:*
-GMP_URL := $(GNU)/gmp/gmp-$(GMP_VERSION).tar.bz2
+GMP_URL := https://gmplib.org/download/gmp/gmp-$(GMP_VERSION).tar.xz
 
 ifeq ($(call need_pkg,'gmp >= 6.2.0'),)
 PKGS_FOUND += gmp
 endif
 
-$(TARBALLS)/gmp-$(GMP_VERSION).tar.bz2:
+$(TARBALLS)/gmp-$(GMP_VERSION).tar.xz:
 	$(call download,$(GMP_URL))
 
-.sum-gmp: gmp-$(GMP_VERSION).tar.bz2
+.sum-gmp: gmp-$(GMP_VERSION).tar.xz
 
-gmp: gmp-$(GMP_VERSION).tar.bz2 .sum-gmp
+gmp: gmp-$(GMP_VERSION).tar.xz .sum-gmp
 	$(UNPACK)
 ifdef HAVE_IOS
 	$(APPLY) $(SRC)/gmp/clock_gettime.patch