Allow to set LD_PLUGIN and use it for 'ar' - helps -flto build with clang.

So, it's possible to just add LD_PLUGIN=/usr/lib64/LLVMgold.so to
autogen.input and it works.

Change-Id: I35cdebfc918793c2ff99fd4f6dce9319b459be0e

config_host.mk.in

@@ -299,6 +299,7 @@ export KDE_LIBS=$(gb_SPACE)@KDE_LIBS@
 export KRB5_LIBS=@KRB5_LIBS@
 export LCMS2_CFLAGS=$(gb_SPACE)@LCMS2_CFLAGS@
 export LCMS2_LIBS=$(gb_SPACE)@LCMS2_LIBS@
+export LD_PLUGIN=@LD_PLUGIN@
 @x_LDFLAGS@ export LDFLAGS=@LDFLAGS@
 export LFS_CFLAGS=@LFS_CFLAGS@
 export LIBBASE_JAR=@LIBBASE_JAR@

configure.ac

@@ -5690,6 +5690,11 @@ AC_SUBST(COMPILER_PLUGINS)
 AC_SUBST(CLANGDIR)
 AC_SUBST(CLANGBUILD)
 
+# Plugin to help linker.
+# Add something like LD_PLUGIN=/usr/lib64/LLVMgold.so to your autogen.input.
+# This makes --enable-lto build with clang work.
+AC_SUBST(LD_PLUGIN)
+
 dnl ===================================================================
 dnl Set the MinGW sys-root
 dnl ===================================================================

solenv/gbuild/platform/unxgcc.mk

@@ -198,6 +198,7 @@ define gb_LinkTarget__command_staticlink
 $(call gb_Helper_abbreviate_dirs,\
 	rm -f $(1) && \
 	$(gb_AR) -rsu $(1) \
+		$(if $(LD_PLUGIN),--plugin $(LD_PLUGIN)) \
 		$(foreach object,$(COBJECTS),$(call gb_CObject_get_target,$(object))) \
 		$(foreach object,$(CXXOBJECTS),$(call gb_CxxObject_get_target,$(object))) \
 		$(foreach object,$(ASMOBJECTS),$(call gb_AsmObject_get_target,$(object))) \