package/apr: fix compile for toolchains without thread support

package/apr: fix compile for toolchains without thread support

Do not trust apr_hints.m4 for setting '-D_REENTRANT', set it only for
toolchains with thread support.

Fixes misleading configure error ([1]):

  checking whether int64_t and int use fmt %d... no
  checking whether int64_t and long use fmt %ld... no
  checking whether int64_t and long long use fmt %lld... no
  configure: error: could not determine the string function for int64_t

which results from warning (promoted to error by '-Werror=cpp'), from
config.log:

  configure:24288: checking whether int64_t and long long use fmt %lld
  configure:24320: .../host/bin/arm-linux-gcc -c -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -Os    -Werror -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -DLINUX -D_REENTRANT -D_GNU_SOURCE conftest.c >&5
  In file included from .../host/arm-buildroot-linux-uclibcgnueabihf/sysroot/usr/include/sys/types.h:26:0,
                 from conftest.c:147:
  .../host/arm-buildroot-linux-uclibcgnueabihf/sysroot/usr/include/features.h:218:5: error: #warning requested reentrant code, but thread support was disabled [-Werror=cpp]
   #   warning requested reentrant code, but thread support was disabled
       ^~~~~~~
  cc1: all warnings being treated as errors
  configure:24320: $? = 1
  configure: failed program was:

[1] http://autobuild.buildroot.net/results/4da73982f160305ac2d5d2b4f615fa98eec5a7a3

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>