gcc to clang (#11453)

* gcc to clang

test
```
export CC="sccache "$CC
export CXX="sccache "$CXX
```
darwin build
```
CC=clang
CXX=clang
```

* darwin - > default clang

.gitlab-ci.yml

@@ -165,8 +165,6 @@ build-darwin:
   variables:
     CARGO_TARGET:                  x86_64-apple-darwin
     CARGO_HOME:                    "${CI_PROJECT_DIR}/.cargo"
-    CC:                            gcc
-    CXX:                           g++
   script:
     - scripts/gitlab/build-linux.sh
   tags:

scripts/gitlab/test-linux.sh

@@ -5,8 +5,8 @@ echo "________Running test-linux.sh________"
 set -e # fail on any error
 set -u # treat unset variables as error
 
-export CC="sccache gcc"
-export CXX="sccache g++"
+export CC="sccache "$CC
+export CXX="sccache "$CXX
 FEATURES="json-tests"
 
 OPTIONS="--release"