package/apr: fix musl build

strerror_r on musl always returns an int since its addition back in 2011
with
https://git.musl-libc.org/cgit/musl/commit/src/string/strerror_r.c?id=0b44a0315b47dd8eced9f3b7f31580cf14bbfc01

As a result, setting ac_cv_strerror_r_rc_int to no results in the
following build failure since bump to version 1.7.2 in commit
783cd8d90d37b5e0b59d6f0bfca6667855b2b9e1:

misc/unix/errorcodes.c: In function 'native_strerror':
misc/unix/errorcodes.c:385:9: error: assignment to 'const char *' from 'int' makes pointer from integer without a cast [-Wint-conversion]
  385 |     msg = strerror_r(statcode, buf, bufsize);
      |         ^

Fixes: 783cd8d90d37b5e0b59d6f0bfca6667855b2b9e1
 - http://autobuild.buildroot.org/results/9a42a4427ff64d47da61c731abb99d7585781cdd

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/apr/apr.mk

@@ -39,7 +39,7 @@ APR_CONF_ENV = \
 	ac_cv_sizeof_struct_iovec=8 \
 	ac_cv_sizeof_pid_t=4 \
 	ac_cv_struct_rlimit=yes \
-	ac_cv_strerror_r_rc_int=no \
+	ac_cv_strerror_r_rc_int=$(if $(BR2_TOOLCHAIN_USES_MUSL),yes,no) \
 	ac_cv_o_nonblock_inherited=no \
 	apr_cv_mutex_recursive=yes \
 	apr_cv_epoll=yes \