mk: allow AR override

To ease cross-development Gentoo alows installing only fully
qualified toolchains tools:
    x86_64-pc-linux-gnu-ar
and similar.

The change allow overriding RANLIB variable similar
to existing CC, HOST_CC, LD and friends.

Reported-by: Agostino Sarubbo
Bug: https://bugs.gentoo.org/720700
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

mk/clang.mk

@@ -3,7 +3,8 @@ _INCLUDE_MK_CLANG_=1
 CC?=clang
 RANLIB?=ranlib
 ONELIB=0
-CC_AR=ar q ${LIBAR}
+AR?=ar
+CC_AR=${AR} q ${LIBAR}
 CFLAGS+=-MD
 CFLAGS_INCLUDE=-I
 LDFLAGS_LINK=-l

mk/gcc.mk

@@ -4,7 +4,8 @@ CC?=gcc
 OBJCOPY?=objcopy
 RANLIB?=ranlib
 ONELIB=0
-CC_AR=ar q ${LIBAR}
+AR?=ar
+CC_AR=${AR} q ${LIBAR}
 CFLAGS+=-MD
 CFLAGS_INCLUDE=-I
 LDFLAGS_LINK=-l

mk/tcc.mk

@@ -1,7 +1,7 @@
 CC=tcc
 RANLIB?=ranlib
 ONELIB=0
-AR=ar
+AR?=ar
 CC_AR=${AR} -r ${LIBAR}
 CC_LIB=${CC} -shared -o ${LIBSO}
 CFLAGS_INCLUDE=-I