Fix 'make install' with custom build_bindir

'make install' failed when setting build_bindir since docs/Makefile expected
the julia executable to be under $(JULIAHOME)/usr/bin/.

Makefile

@@ -106,7 +106,7 @@ julia-debug julia-release : julia-% : julia-ui-% julia-sysimg-% julia-symlink ju
 debug release : % : julia-%
 
 docs: julia-sysimg-$(JULIA_BUILD_MODE)
- @$(MAKE) $(QUIET_MAKE) -C $(BUILDROOT)/doc
+ @$(MAKE) $(QUIET_MAKE) -C $(BUILDROOT)/doc JULIA_EXECUTABLE=$(JULIA_EXECUTABLE_$(JULIA_BUILD_MODE))
 
 check-whitespace:
 ifneq ($(NO_GIT), 1)

doc/Makefile

@@ -5,7 +5,8 @@ default: html
 # You can set these variables from the command line.
 SRCDIR := $(abspath $(dir $(lastword $(MAKEFILE_LIST))))
 JULIAHOME := $(abspath $(SRCDIR)/..)
-JULIA_EXECUTABLE := $(JULIAHOME)/usr/bin/julia
+include $(JULIAHOME)/Make.inc
+JULIA_EXECUTABLE := $(build_bindir)/julia
 
 .PHONY: help clean cleanall html pdf linkcheck doctest check