drop -Werror from default build flags

[adammoody]

One of our users tripped on a -Werror message.

axl_sync.c:39:13 error: variable 'success' set but not used [-Werror=unused-bug-set-variable]
  int success;

This PR disables those to make life easier for end users building with newer compilers that may be more strict.

We should use these flags in our travis/testing/development builds, but let's otherwise make life easier for our end users.

[tonyhutter]

AXL builds normally with gcc 10.2, which is the newest stable. The user is probably building with cmake -DCMAKE_BUILD_TYPE=Release .. which disables the asserts(). It's getting hung up on variables that are declared and only used for asserts:

Scanning dependencies of target axl_o
[  7%] Building C object src/CMakeFiles/axl_o.dir/axl.c.o
/home/hutter2/AXL/src/axl.c: In function ‘AXL_Config_Set’:
/home/hutter2/AXL/src/axl.c:399:31: error: unused variable ‘kvtree_first_elem_hash’ [-Werror=unused-variable]
  399 |                 const kvtree* kvtree_first_elem_hash =
      |                               ^~~~~~~~~~~~~~~~~~~~~~
/home/hutter2/AXL/src/axl.c:438:23: error: unused variable ‘kvtree_first_elem_hash’ [-Werror=unused-variable]
  438 |         const kvtree* kvtree_first_elem_hash =
      |                       ^~~~~~~~~~~~~~~~~~~~~~
/home/hutter2/AXL/src/axl.c: In function ‘__AXL_Dispatch’:
/home/hutter2/AXL/src/axl.c:953:9: error: unused variable ‘success’ [-Werror=unused-variable]
  953 |     int success = kvtree_util_get_int(file_list, AXL_KEY_CONFIG_MKDIR,
      |         ^~~~~~~
cc1: all warnings being treated as errors
make[2]: *** [src/CMakeFiles/axl_o.dir/build.make:82: src/CMakeFiles/axl_o.dir/axl.c.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:979: src/CMakeFiles/axl_o.dir/all] Error 2
make: *** [Makefile:160: all] Error 2

My vote would be to fix these build issues and leave -Wall on, but I'll bow to @adammoody for the final say.