Fix name mangling issues.

Move je_* definitions from jemalloc_macros.h.in to jemalloc_defs.h.in,
because only the latter is an autoconf header (#undef substitution
occurs).

Fix unit tests to use automatic mangling, so that e.g. mallocx is
macro-substituted to becom jet_mallocx.

test/include/test/jemalloc_test.h.in

@@ -64,6 +64,7 @@
  */
 #ifdef JEMALLOC_UNIT_TEST
 #  define JEMALLOC_JET
+#  define JEMALLOC_MANGLE
 #  include "jemalloc/internal/jemalloc_internal.h"
 
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