package/dmalloc: fix strdup detection

Fix the following build failure raised since bump to version 5.6.5 in
commit 736932c71552adc4dfa95c5236d1064860900e8a:

In file included from /home/buildroot/autobuild/instance-0/output-1/host/arm-buildroot-linux-gnueabi/sysroot/usr/include/string.h:631:0,
                 from arg_check.c:31:
dmalloc.h:377:7: error: expected identifier or '(' before '__extension__'
 char *strdup(const char *string);
       ^

This build failure is raised because 0004-fix-strdup.patch was wrongly
removed and because configure.ac wrongly assumes that strdup macro is
not available when cross-compiling

As configure.ac can't be patched because autoreconf is not working,
patch configure as already done in DMALLOC_POST_PATCH. An upstream patch
on configure.ac will be sent as soon as upstream merges this old PR:
https://github.com/j256/dmalloc/pull/44

Fixes:
 - http://autobuild.buildroot.org/results/00565c4b0618ea5f553d8c22284637b6574e4a93

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/dmalloc/dmalloc.mk

@@ -48,6 +48,7 @@ DMALLOC_CONF_ENV = CFLAGS="$(DMALLOC_CFLAGS)"
 
 define DMALLOC_POST_PATCH
 	$(SED) 's/^ac_cv_page_size=0$$/ac_cv_page_size=12/' $(@D)/configure
+	$(SED) 's/ac_cv_strdup_macro=no$$/ac_cv_strdup_macro=yes/' $(@D)/configure
 	$(SED) 's/(ld -/($${LD-ld} -/' $(@D)/configure
 	$(SED) 's/'\''ld -/"$${LD-ld}"'\'' -/' $(@D)/configure
 	$(SED) 's/ar cr/$$(AR) cr/' $(@D)/Makefile.in