diff --git a/CMakeLists.txt b/CMakeLists.txt
index 4e07bbd8..9af5f637 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -23,14 +23,15 @@ list(APPEND CMAKE_MODULE_PATH ${CMAKE_SOURCE_DIR}/cmake)
 # declaration (no members) and rejects base-member access. So prefer GCC.
 # (GCC does hard-error on the one struct that declares a member with the same
 # name as an inherited one — CCTVRecord's `pad` — see docs/internals.md.)
-# An explicit -DCMAKE_C_COMPILER=... always wins.
-if(NOT DEFINED CMAKE_C_COMPILER)
+# An explicit -DCMAKE_C_COMPILER=... or a CC environment variable always wins.
+if(NOT DEFINED CMAKE_C_COMPILER AND "$ENV{CC}" STREQUAL "")
   if(CMAKE_HOST_APPLE)
-    # Homebrew installs real GNU GCC with a version suffix; /usr/bin/gcc is
-    # Apple Clang and does not support the Plan 9 struct extensions required
-    # by the decomp's `inherits` pattern.
+    # Homebrew (gcc-N) and MacPorts (gcc-mp-N) install real GNU GCC with a
+    # version suffix; /usr/bin/gcc is Apple Clang and does not support the
+    # Plan 9 struct extensions required by the decomp's `inherits` pattern.
     find_program(_GE007_GCC
-      NAMES gcc-16 gcc-15 gcc-14 gcc-13 gcc-12 gcc-11 gcc)
+      NAMES gcc-16 gcc-mp-16 gcc-15 gcc-mp-15 gcc-14 gcc-mp-14 gcc-13 gcc-mp-13
+            gcc-12 gcc-mp-12 gcc-11 gcc-mp-11 gcc)
   else()
     find_program(_GE007_GCC NAMES gcc)
   endif()
@@ -39,7 +40,8 @@ if(NOT DEFINED CMAKE_C_COMPILER)
     get_filename_component(_GE007_GCC_DIR "${_GE007_GCC}" DIRECTORY)
     if(CMAKE_HOST_APPLE)
       find_program(_GE007_GXX
-        NAMES g++-16 g++-15 g++-14 g++-13 g++-12 g++-11 g++
+        NAMES g++-16 g++-mp-16 g++-15 g++-mp-15 g++-14 g++-mp-14 g++-13 g++-mp-13
+              g++-12 g++-mp-12 g++-11 g++-mp-11 g++
         PATHS "${_GE007_GCC_DIR}" NO_DEFAULT_PATH)
     else()
       find_program(_GE007_GXX NAMES g++
diff --git a/docs/building.md b/docs/building.md
index 60444a8d..63bd6dd9 100644
--- a/docs/building.md
+++ b/docs/building.md
@@ -55,7 +55,19 @@ brew install cmake gcc sdl2 zlib python3
 
 The project requires real GNU GCC for the decomp's Plan 9 struct extensions.
 Apple's `/usr/bin/gcc` is Clang and is not compatible. CMake automatically
-selects Homebrew's versioned `gcc-N`/`g++-N` executables.
+selects Homebrew's versioned `gcc-N`/`g++-N` (or MacPorts' `gcc-mp-N`/`g++-mp-N`)
+executables; set `CC`/`CXX` to pick a specific one. Delete the build directory
+after switching compilers.
+
+If GCC fails with `fatal error: _bounds.h: No such file or directory`, it was
+built against a newer macOS SDK than the one CMake picked (the `xcode-select`
+default). Point `SDKROOT` at the SDK GCC was built with (`gcc -v` shows it as
+`--with-sysroot`), for example with MacPorts GCC 15:
+
+```sh
+SDKROOT=/Library/Developer/CommandLineTools/SDKs/MacOSX15.sdk \
+CC=/opt/local/bin/gcc-mp-15 CXX=/opt/local/bin/g++-mp-15 ./build-pc.sh ntsc-final
+```
 
 ### Asset extraction (decompilation toolchain)