make: Add __BASH_MAKE_COMPLETION__ variable

This variable is there so that makefiles can avoid adding targets that shouldn't be offered.
A typical example would be targets added to dependency files by the -MP option to gcc.

completions/make

@@ -144,7 +144,8 @@ _make()
 
         local reset=$( set +o | command grep -F posix ); set +o posix # <(...)
         COMPREPLY=( $( LC_ALL=C \
-            make -npq "${makef[@]}" "${makef_dir[@]}" .DEFAULT 2>/dev/null | \
+            make -npq __BASH_MAKE_COMPLETION__=1 \
+                "${makef[@]}" "${makef_dir[@]}" .DEFAULT 2>/dev/null | \
             sed -nf <(_make_target_extract_script $mode "$cur") ) )
         $reset
 

test/fixtures/make/Makefile

@@ -1,5 +1,6 @@
 $(info confuse: make)
 
+CFLAGS=-MMD -MP
 NAME := sample
 
 .PHONY: all
@@ -16,3 +17,7 @@ install: all
 .PHONY: clean
 clean:
 	-rm -f $(NAME)
+
+ifndef __BASH_MAKE_COMPLETION__
+-include sample.d
+endif

test/fixtures/make/sample.c

@@ -1,4 +1,5 @@
 #include <stdio.h>
+#include "sample.h"
 
 int main(void)
 {

test/fixtures/make/sample.d

@@ -0,0 +1,3 @@
+sample.o: sample.c sample.h
+
+sample.h: