Explicitly include our m4 files first. This might minimize the impact

that other package's underquoted m4 function definitions have on ours.

configure.ac

@@ -1,5 +1,13 @@
 AC_PREREQ(2.57)
 
+dnl Explicitly include our m4 files first. Without these explicit inclusions
+dnl aclocal will actually find and include them automatically, but before it
+dnl does it might fully scan all other system installed m4 files. Doing this
+dnl explicit inclusion also allows us to override definitions of macros done
+dnl elsewhere in other packages with the ones done in our included files.
+m4_include([m4/reentrant.m4])
+m4_include([acinclude.m4])
+
 dnl Version not hardcoded here. Fetched later from ares_version.h
 AC_INIT([c-ares], [-],
   [c-ares mailing list => http://cool.haxx.se/mailman/listinfo/c-ares])