amalgam.rb: support gems with core-affecting defines

Gems like mruby-task add preprocessor defines (MRB_USE_TASK_SCHEDULER)
that affect mrb_state structure. The amalgamation generator now detects
these defines from the build configuration and adds them at the top of
mruby.h before struct definitions are encountered.

Supported define patterns: MRB_USE_*, MRB_UTF8_*, HAVE_MRUBY_*

Co-authored-by: Claude <noreply@anthropic.com>

doc/guides/amalgamation.md

@@ -72,6 +72,7 @@ The following gems work with amalgamation:
 - `mruby-enum-ext`, `mruby-compar-ext`
 - `mruby-error`, `mruby-math`, `mruby-struct`, `mruby-bigint`
 - `mruby-io` (with `hal-posix-io`)
+- `mruby-task` (with `hal-posix-task`)
 
 ### Excluded Gems
 
@@ -79,10 +80,6 @@ Binary gems (`mruby-bin-*`) are automatically excluded as they contain
 their own `main()` function. The amalgamation produces a library, not
 an executable.
 
-### Incompatible Gems
-
-- `mruby-task` - Requires special build configuration (`mrb->task` member)
-
 ## Example Configuration
 
 A minimal configuration for amalgamation:
@@ -130,6 +127,12 @@ Typical sizes depend on included gems:
 - Local includes (`.cstub` files) are automatically inlined
 - Generated files (`mrblib.c`, `gem_init.c`) are included
 
+### Gem Defines
+
+Gems that add preprocessor defines affecting core structures (like
+`MRB_USE_TASK_SCHEDULER` from mruby-task) are automatically detected
+and included at the top of `mruby.h`.
+
 ### Build Order
 
 1. Core sources (`src/*.c`)

lib/mruby/amalgam.rb

@@ -169,6 +169,26 @@ module MRuby
         #include <string.h>
 
       PREAMBLE
+
+      # Add build-level defines from gems (e.g., MRB_USE_TASK_SCHEDULER)
+      gem_defines = collect_gem_defines
+      unless gem_defines.empty?
+        f.puts "/* Gem-required defines */"
+        gem_defines.each do |d|
+          f.puts "#define #{d}"
+        end
+        f.puts
+      end
+    end
+
+    # Collect defines added by gems that affect core headers
+    def collect_gem_defines
+      defines = []
+      @build.defines.each do |d|
+        # Include defines that affect mrb_state or core functionality
+        defines << d if d =~ /^MRB_USE_|^MRB_UTF8_|^HAVE_MRUBY_/
+      end
+      defines.uniq.sort
     end
 
     def write_header_postamble(f)