macos not build （error: incompatible pointer types passing '_Atomic(double) *' to parameter of type 'double）

[fengve]

/Users/fengwei/prometheus-client-c/prom/src/prom_metric_sample.c:69:54: error: incompatible pointer types passing
'_Atomic(double) *' to parameter of type 'double *' [-Werror,-Wincompatible-pointer-types]
if (atomic_compare_exchange_weak(&self->r_value, &old, new)) {
^~~~
/Library/Developer/CommandLineTools/usr/lib/clang/11.0.0/include/stdatomic.h:143:108: note: expanded from macro
'atomic_compare_exchange_weak'
#define atomic_compare_exchange_weak(object, expected, desired) __c11_atomic_compare_exchange_weak(object, expected, des...
^~~~~~~~
/Users/fengwei/prometheus-client-c/prom/src/prom_metric_sample.c:84:54: error: incompatible pointer types passing
'_Atomic(double) *' to parameter of type 'double *' [-Werror,-Wincompatible-pointer-types]
if (atomic_compare_exchange_weak(&self->r_value, &old, new)) {
^~~~
/Library/Developer/CommandLineTools/usr/lib/clang/11.0.0/include/stdatomic.h:143:108: note: expanded from macro
'atomic_compare_exchange_weak'
#define atomic_compare_exchange_weak(object, expected, desired) __c11_atomic_compare_exchange_weak(object, expected, des...

--------------------clang version--------------------------------
Apple clang version 11.0.0 (clang-1100.0.33.8)
Target: x86_64-apple-darwin19.6.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

---------------os--------------
Darwin Kernel Version 19.6.0: Mon Aug 31 22:12:52 PDT 2020; root:xnu-6153.141.2~1/RELEASE_X86_64 x86_64

[czheji]

atomic_load should return a bare type, which should be corrected to:
double old = atomic_load(&self->r_value);

[eakraly]

Fix in #71