Revert "compat/strdup.h: move common compat check for strdup() to own file"

This reverts commit aaba8c1080.

This commit breaks builddir != srcdir build, but, more importantly, it
also adds a dependency on a header, "config.h", which is not installed
and it's supposed to be private — since it's generated at configuration
time and it's not meant to be used by projects compiling against a
library.

Makefile.am

@@ -18,7 +18,6 @@ libjson_cincludedir = $(includedir)/json-c
 libjson_cinclude_HEADERS = \
 	arraylist.h \
 	bits.h \
-	compat/strdup.h \
 	debug.h \
 	json.h \
 	json_c_version.h \