configure: Refactor toolchain flag setting

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

configure

@@ -2718,44 +2718,43 @@ echo "# $0 $LIBAV_CONFIGURATION" > $logfile
 set >> $logfile
 
 case "$toolchain" in
-    clang-asan)
-        cc_default="clang"
+    *-asan)
+        cc_default="${toolchain%-asan}"
         add_cflags  -fsanitize=address
         add_ldflags -fsanitize=address
     ;;
-    clang-tsan)
-        cc_default="clang"
+    *-tsan)
+        cc_default="${toolchain%-tsan}"
         add_cflags  -fsanitize=thread -pie
         add_ldflags -fsanitize=thread -pie
-    ;;
-    clang-usan)
-        cc_default="clang"
-        add_cflags  -fsanitize=undefined -O1
-        add_ldflags -fsanitize=undefined
-    ;;
-    gcc-asan)
-        cc_default="gcc"
-        add_cflags  -fsanitize=address
-        add_ldflags -fsanitize=address
-    ;;
+        case "$toolchain" in
             gcc-tsan)
-        cc_default="gcc"
-        add_cflags  -fsanitize=thread -pie -fPIC
-        add_ldflags -fsanitize=thread -pie -fPIC
+                add_cflags  -fPIC
+                add_ldflags -fPIC
                 ;;
-    gcc-usan)
-        cc_default="gcc"
+        esac
+    ;;
+    *-usan)
+        cc_default="${toolchain%-usan}"
         add_cflags  -fsanitize=undefined
         add_ldflags -fsanitize=undefined
+        case "$toolchain" in
+            clang-usan)
+                add_cflags -O1
                 ;;
-    valgrind-massif)
+        esac
+    ;;
+    valgrind-*)
         target_exec_default="valgrind"
+        case "$toolchain" in
+            valgrind-massif)
                 target_exec_args="--alloc-fn=av_malloc --alloc-fn=av_mallocz"
                 ;;
             valgrind-memcheck)
-        target_exec_default="valgrind"
                 target_exec_args="--track-origins=yes --leak-check=full"
                 ;;
+        esac
+    ;;
     msvc)
         # Check whether the current MSVC version needs the C99 converter.
         # From MSVC 2013 (compiler major version 18) onwards, it does actually