don't override libc++

.github/.libcxx-setup.sh

diff --git a/.github/.libcxx-setup.sh b/.github/.libcxx-setup.sh
index 5600840..ba278cf 100755
--- a/.github/.libcxx-setup.sh
+++ b/.github/.libcxx-setup.sh
@@ -13,7 +13,7 @@
 cmake -DCMAKE_C_COMPILER=${C_COMPILER}          \
       -DCMAKE_CXX_COMPILER=${COMPILER}          \
       -DCMAKE_BUILD_TYPE=RelWithDebInfo         \
-      -DCMAKE_INSTALL_PREFIX=/usr               \
+      -DCMAKE_INSTALL_PREFIX=/usr/local         \
       -DLIBCXX_ABI_UNSTABLE=OFF                 \
       -DLLVM_USE_SANITIZER=${LIBCXX_SANITIZER}  \
       -DLLVM_BUILD_32_BITS=${BUILD_32_BITS}     \

.github/workflows/sanitizer.yml

diff --git a/.github/workflows/sanitizer.yml b/.github/workflows/sanitizer.yml
index 57470ba..e0df345 100644
--- a/.github/workflows/sanitizer.yml
+++ b/.github/workflows/sanitizer.yml
@@ -7,6 +7,7 @@
 env:
   CC: clang
   CXX: clang++
+  EXTRA_FLAGS: "-g -O2 -fno-omit-frame-pointer -L/usr/local/lib -I/usr/local/include -I/usr/local/include/c++/v1"
   EXTRA_CXX_FLAGS: "-stdlib=libc++"
   UBSAN_OPTIONS: "print_stacktrace=1"
 
@@ -25,25 +26,25 @@
     - name: configure msan env
       if: matrix.sanitizer == 'msan'
       run: |
-        echo "EXTRA_FLAGS=-g -O2 -fno-omit-frame-pointer -fsanitize=memory -fsanitize-memory-track-origins" >> $GITHUB_ENV
+        echo "EXTRA_FLAGS=$EXTRA_FLAGS -fsanitize=memory -fsanitize-memory-track-origins" >> $GITHUB_ENV
         echo "LIBCXX_SANITIZER=MemoryWithOrigins" >> $GITHUB_ENV
 
     - name: configure ubsan env
       if: matrix.sanitizer == 'ubsan'
       run: |
-        echo "EXTRA_FLAGS=-g -O2 -fno-omit-frame-pointer -fsanitize=undefined -fno-sanitize-recover=all" >> $GITHUB_ENV
+        echo "EXTRA_FLAGS=$EXTRA_FLAGS -fsanitize=undefined -fno-sanitize-recover=all" >> $GITHUB_ENV
         echo "LIBCXX_SANITIZER=Undefined" >> $GITHUB_ENV
 
     - name: configure asan env
       if: matrix.sanitizer == 'asan'
       run: |
-        echo "EXTRA_FLAGS=-g -O2 -fno-omit-frame-pointer -fsanitize=address -fno-sanitize-recover=all" >> $GITHUB_ENV
+        echo "EXTRA_FLAGS=$EXTRA_FLAGS -fsanitize=address -fno-sanitize-recover=all" >> $GITHUB_ENV
         echo "LIBCXX_SANITIZER=Address" >> $GITHUB_ENV
 
     - name: configure tsan env
       if: matrix.sanitizer == 'tsan'
       run: |
-        echo "EXTRA_FLAGS=-g -O2 -fno-omit-frame-pointer -fsanitize=thread -fno-sanitize-recover=all" >> $GITHUB_ENV
+        echo "EXTRA_FLAGS=$EXTRA_FLAGS -fsanitize=thread -fno-sanitize-recover=all" >> $GITHUB_ENV
         echo "LIBCXX_SANITIZER=Thread" >> $GITHUB_ENV
 
     - name: install llvm stuff