CI: Build VTest with clang

Current VTest master fails to build using gcc, see vtest/VTest#27.

This patch is to be reverted once VTest is fixed.
diff --git a/.github/workflows/vtest.yml b/.github/workflows/vtest.yml
index cb52f27..1d62f98 100644
--- a/.github/workflows/vtest.yml
+++ b/.github/workflows/vtest.yml
@@ -63,7 +63,7 @@
         curl -fsSL https://github.com/vtest/VTest/archive/master.tar.gz -o VTest.tar.gz
         mkdir VTest
         tar xvf VTest.tar.gz -C VTest --strip-components=1
-        make -C VTest -j$(nproc) FLAGS="-O2 -s -Wall"
+        make -C VTest -j$(nproc) FLAGS="-O2 -s -Wall" CC=clang
         sudo install -m755 VTest/vtest /usr/local/bin/vtest
     - name: Install SSL ${{ matrix.ssl }}
       if: ${{ matrix.ssl && matrix.ssl != 'stock' }}
diff --git a/.travis.yml b/.travis.yml
index 1aa415a..37b667b 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -40,7 +40,7 @@
 install:
   - git clone https://github.com/VTest/VTest.git ../vtest
   # Special flags due to: https://github.com/vtest/VTest/issues/12
-  - make -C ../vtest FLAGS="-O2 -s -Wall"
+  - make -C ../vtest FLAGS="-O2 -s -Wall" CC=clang
 
 script:
   - make -j$(nproc) ERR=1 TARGET=linux-glibc CC=$CC DEBUG=-DDEBUG_STRICT=1 $FLAGS