fix(thorvg): expand response files in compiler flags for meson

ESP-IDF's latest version uses response files (arguments starting with @)
to handle long compiler flag lists. These response file references are
embedded in CMAKE_CXX_FLAGS and CMAKE_EXE_LINKER_FLAGS.

When passed directly to meson's cross-file, these references fail because:
1. The response files may not exist when meson runs
2. Meson cannot access these files from its build context
3. The paths may be invalid from meson's working directory

This commit adds a function to expand response file references before
passing flags to meson. If a response file exists, its contents are
read and expanded inline. If it doesn't exist, it's skipped (assuming
essential flags are provided directly).

Fixes the build failure:
ERROR: Compiler cannot compile programs.
riscv32-esp-elf-g++: error: @"...toolchain/cxxflags": linker input
file not found: No such file or directory

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: mahavirj

thorvg/CMakeLists.txt

@@ -59,6 +59,41 @@ set(compiler_args ${CMAKE_CXX_FLAGS})
 separate_arguments(compiler_args)
 list(FILTER compiler_args EXCLUDE REGEX "^$")
 
+# Expand response files (arguments starting with @"filename")
+# ESP-IDF's latest version uses response files for compiler flags
+# These need to be expanded because meson cannot read them from its build context
+function(expand_response_files arg_list output_var)
+    set(expanded_args)
+    foreach(arg IN LISTS arg_list)
+        # Check if argument is a response file reference: @"filename" or @filename
+        if(arg MATCHES "^@\"?(.+)\"?$")
+            set(response_file "${CMAKE_MATCH_1}")
+            # Remove quotes if present
+            string(REGEX REPLACE "^\"(.+)\"$" "\\1" response_file "${response_file}")
+            # Check if file exists and read its contents
+            if(EXISTS "${response_file}")
+                file(READ "${response_file}" file_contents)
+                # Split file contents into individual arguments
+                string(REGEX REPLACE "[\r\n]+" ";" file_args "${file_contents}")
+                separate_arguments(file_args)
+                list(FILTER file_args EXCLUDE REGEX "^$")
+                list(APPEND expanded_args ${file_args})
+                message(STATUS "Expanded response file ${response_file} with ${CMAKE_MATCH_0} arguments")
+            else()
+                # Response file doesn't exist yet (created at build time)
+                # Skip it - essential flags should be provided directly
+                message(STATUS "Skipping non-existent response file: ${response_file}")
+            endif()
+        else()
+            list(APPEND expanded_args "${arg}")
+        endif()
+    endforeach()
+    set(${output_var} ${expanded_args} PARENT_SCOPE)
+endfunction()
+
+expand_response_files("${compiler_args}" compiler_args)
+expand_response_files("${linker_args}" linker_args)
+
 message(STATUS "CMAKE_EXE_LINKER_FLAGS: ${linker_args}")
 message(STATUS "CMAKE_CXX_FLAGS: ${compiler_args}")