Disable Valgrind by default during testing (#3269)

Previously, the configure script automatically enables Valgrind during
testing when the valgrind command is found. However, running tests with
Valgrind is quite slow, so it is better to disable it by default, and
to enable it only when the user specifies the `--enable-valgrind` flag.

Makefile.am

@@ -65,23 +65,18 @@ endif
 
 include_HEADERS = src/jv.h src/jq.h
 
-if ENABLE_UBSAN
-AM_CFLAGS += -fsanitize=undefined
-endif
-
 AM_CPPFLAGS = -I$(srcdir)/src -I$(srcdir)/vendor
 
-### Running tests under Valgrind
+### Address sanitizer (ASan)
 
 if ENABLE_ASAN
 AM_CFLAGS += -fsanitize=address
-NO_VALGRIND = 1
-else
-if ENABLE_VALGRIND
-NO_VALGRIND =
-else
-NO_VALGRIND = 1
 endif
+
+### Undefined Behavior Sanitizer
+
+if ENABLE_UBSAN
+AM_CFLAGS += -fsanitize=undefined
 endif
 
 ### Code coverage with gcov
@@ -127,7 +122,11 @@ TESTS = tests/mantest tests/jqtest tests/shtest tests/utf8test tests/base64test
 if !WIN32
 TESTS += tests/optionaltest
 endif
-AM_TESTS_ENVIRONMENT = JQ=$(abs_builddir)/jq NO_VALGRIND=$(NO_VALGRIND)
+
+AM_TESTS_ENVIRONMENT = JQ=$(abs_builddir)/jq
+if ENABLE_VALGRIND
+AM_TESTS_ENVIRONMENT += ENABLE_VALGRIND=1
+endif
 
 # This is a magic make variable that causes it to treat tests/man.test as a
 # DATA-type dependency for the check target. As a result, it will attempt to