build-system: turn off lto=thin when building the fuzzers

With lto=thin the fuzzers fail as soon as they start with
```
ERROR: The size of coverage PC tables does not match the
number of instrumented PCs. This might be a compiler bug,
please contact the libFuzzer developers.
Also check https://bugs.llvm.org/show_bug.cgi?id=34636
for possible workarounds (tl;dr: don't use the old GNU ld)
```

Signed-off-by: Evgeny Vereshchagin <evvers@ya.ru>

configure.ac

@@ -484,12 +484,16 @@ else
 	AC_MSG_RESULT([no])
 fi
 
-if test "x$enable_fuzzers" = "xyes" -a "x$LIB_FUZZING_ENGINE" = x; then
-	CC_CHECK_FLAGS_APPEND([AM_CFLAGS],[CFLAGS],[ \
-		-DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION \
-		-fsanitize=fuzzer-no-link])
-	AC_SUBST(AM_CFLAGS)
+if test "x$enable_fuzzers" = "xyes"; then
+	if test "x$LIB_FUZZING_ENGINE" = x; then
+		CC_CHECK_FLAGS_APPEND([AM_CFLAGS],[CFLAGS],[ \
+			-DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION \
+			-fsanitize=fuzzer-no-link])
+	fi
+else
+	CC_CHECK_FLAGS_APPEND([AM_CFLAGS],[CFLAGS],[-flto=thin])
 fi
+AC_SUBST(AM_CFLAGS)
 
 # Optional test binaries
 AC_ARG_ENABLE([tests],
@@ -815,7 +819,6 @@ CC_CHECK_FLAGS_APPEND([AM_CFLAGS],[CFLAGS],[ \
 	-Warray-bounds \
 	-Wrestrict \
 	-Wreturn-local-addr \
-	-flto=thin \
 	-fsanitize=cfi \
 	-Wstringop-overflow])
 AC_SUBST(AM_CFLAGS)

src/tests/oss-fuzz.sh

@@ -24,9 +24,6 @@ mkdir -p $OUT
 
 export LIB_FUZZING_ENGINE=${LIB_FUZZING_ENGINE:--fsanitize=fuzzer}
 
-# AFL++ and hoggfuzz are both incompatible with lto=thin apparently
-sed -i '/-flto=thin/d' configure.ac
-
 # turn off the libutil dependency
 sed -i 's/^AC_CHECK_LIB(util/#/' configure.ac