m4: remove equal sign from define directive in ax_valgrind_check.m4

m4: remove equal sign from define directive in ax_valgrind_check.m4

Support for providing equal sign in define directives has been added
only in GNU Make 3.82 [1] and it provides the same semantics (variables
should be recursively expanded) as when it is omitted at all, so let's
remove it in order to preserve compatibility with older GNU Make
versions (like the one used on Travis).

[1] https://git.savannah.gnu.org/cgit/make.git/tree/ChangeLog?h=3.82#n766

* m4/ax_valgrind_check.m4 <define valgrind_tool_rule>: Remove equal
sign.