Meta: Allow Clang to be used as the host compiler for Lagom

Various Clang binaries are now considered when choosing the compiler for
Lagom.

The selection precedence is as follows:
1. Use the compiler set via CC/CXX if it's a supported version
2. Use newest available GCC if it's supported
3. Use newest available Clang if it's supported

Note that Apple Clang is still not supported, as its versioning scheme
and the fact that it masquerades as both GCC and Clang would complicate
this logic even more.

Fixes #12253

Meta/serenity.sh

@@ -89,6 +89,7 @@ else
 fi
 
 CMAKE_ARGS=()
+HOST_COMPILER=""
 
 # Toolchain selection only applies to non-lagom targets.
 if [ "$TARGET" != "lagom" ]; then
@@ -141,37 +142,77 @@ create_build_dir() {
     fi
 }
 
-pick_gcc() {
+is_supported_compiler() {
+    local COMPILER="$1"
+    if [ -z "$COMPILER" ]; then
+        return 1
+    fi
+
+    local VERSION=""
+    VERSION="$($COMPILER -dumpversion)" || return 1
+    local MAJOR_VERSION=""
+    MAJOR_VERSION="${VERSION%%.*}"
+    if $COMPILER --version 2>&1 | grep "Apple clang" >/dev/null; then
+        return 1
+    elif $COMPILER --version 2>&1 | grep "clang" >/dev/null; then
+        [ "$MAJOR_VERSION" -gt 12 ] && return 0
+    else
+        [ "$MAJOR_VERSION" -gt 10 ] && return 0
+    fi
+    return 1
+}
+
+find_newest_compiler() {
     local BEST_VERSION=0
-    local BEST_GCC_CANDIDATE=""
+    local BEST_CANDIDATE=""
-    for GCC_CANDIDATE in egcc gcc gcc-11 gcc-12 /usr/local/bin/gcc-11 /opt/homebrew/bin/gcc-11; do
+    for CANDIDATE in "$@"; do
-        if ! command -v $GCC_CANDIDATE >/dev/null 2>&1; then
+        if ! command -v "$CANDIDATE" >/dev/null 2>&1; then
             continue
         fi
-        if $GCC_CANDIDATE --version 2>&1 | grep "Apple clang" >/dev/null; then
+        if $CANDIDATE --version 2>&1 | grep "Apple clang" >/dev/null; then
             continue
         fi
-        if ! $GCC_CANDIDATE -dumpversion >/dev/null 2>&1; then
+        if ! $CANDIDATE -dumpversion >/dev/null 2>&1; then
             continue
         fi
         local VERSION=""
-        VERSION="$($GCC_CANDIDATE -dumpversion)"
+        VERSION="$($CANDIDATE -dumpversion)"
         local MAJOR_VERSION="${VERSION%%.*}"
         if [ "$MAJOR_VERSION" -gt "$BEST_VERSION" ]; then
             BEST_VERSION=$MAJOR_VERSION
-            BEST_GCC_CANDIDATE="$GCC_CANDIDATE"
+            BEST_CANDIDATE="$CANDIDATE"
         fi
     done
-    CMAKE_ARGS+=("-DCMAKE_C_COMPILER=$BEST_GCC_CANDIDATE")
+    HOST_COMPILER=$BEST_CANDIDATE
-    CMAKE_ARGS+=("-DCMAKE_CXX_COMPILER=${BEST_GCC_CANDIDATE/gcc/g++}")
+}
-    if [ "$BEST_VERSION" -lt 11 ]; then
+
-        die "Please make sure that GCC version 11 or higher is installed."
+pick_host_compiler() {
+    if is_supported_compiler "$CC" && is_supported_compiler "$CXX"; then
+        CMAKE_ARGS+=("-DCMAKE_C_COMPILER=$CC")
+        CMAKE_ARGS+=("-DCMAKE_CXX_COMPILER=$CXX")
+        return
     fi
+
+    find_newest_compiler egcc gcc gcc-11 gcc-12 /usr/local/bin/gcc-11 /opt/homebrew/bin/gcc-11
+    if is_supported_compiler "$HOST_COMPILER"; then
+        CMAKE_ARGS+=("-DCMAKE_C_COMPILER=$HOST_COMPILER")
+        CMAKE_ARGS+=("-DCMAKE_CXX_COMPILER=${HOST_COMPILER/gcc/g++}")
+        return
+    fi
+
+    find_newest_compiler clang clang-13 clang-14 clang-15
+    if is_supported_compiler "$HOST_COMPILER"; then
+        CMAKE_ARGS+=("-DCMAKE_C_COMPILER=$HOST_COMPILER")
+        CMAKE_ARGS+=("-DCMAKE_CXX_COMPILER=${HOST_COMPILER/clang/clang++}")
+        return
+    fi
+
+    die "Please make sure that GCC version 11, Clang version 13, or higher is installed."
 }
 
 cmd_with_target() {
     is_valid_target || ( >&2 echo "Unknown target: $TARGET"; usage )
-    pick_gcc
+    pick_host_compiler
 
     if [ ! -d "$SERENITY_SOURCE_DIR" ]; then
         SERENITY_SOURCE_DIR="$(get_top_dir)"