add cmake option to add C pre processor definitions more easily

CMakeLists.txt

@@ -31,6 +31,7 @@ option(MI_SKIP_COLLECT_ON_EXIT "Skip collecting memory on program exit" OFF)
 option(MI_NO_PADDING        "Force no use of padding even in DEBUG mode etc." OFF)
 option(MI_INSTALL_TOPLEVEL  "Install directly into $CMAKE_INSTALL_PREFIX instead of PREFIX/lib/mimalloc-version" OFF)
 option(MI_NO_THP            "Disable transparent huge pages support on Linux/Android for the mimalloc process only" OFF)
+option(MI_EXTRA_CPPDEFS     "Extra pre-processor definitions (use as `-DMI_EXTRA_CPPDEFS=\"opt1=val1;opt2=val2\"`)" "")
 
 # deprecated options
 option(MI_CHECK_FULL        "Use full internal invariant checking in DEBUG mode (deprecated, use MI_DEBUG_FULL instead)" OFF)
@@ -62,8 +63,8 @@ set(mi_sources
 set(mi_cflags "")
 set(mi_cflags_static "")            # extra flags for a static library build
 set(mi_cflags_dynamic "")           # extra flags for a shared-object library build
-set(mi_defines "")
 set(mi_libraries "")
+set(mi_defines ${MI_EXTRA_CPPDEFS})
 
 # -----------------------------------------------------------------------------
 # Convenience: set default build type depending on the build directory