meson: Add option to print preprocessor defines

[mr-c]

The diff below is from a current test branch, but I'm not merging it as it the list is long and noisy; it shouldn't be on all the time

diff --git a/test/meson.build b/test/meson.build
index afe8afaf..309b62b2 100644
--- a/test/meson.build
+++ b/test/meson.build
@@ -71,3 +71,9 @@ subdir('x86')
 subdir('arm')
 subdir('wasm')
 subdir('mips')
+
+if cc.get_id() == 'gcc' or cc.get_id() == 'clang'
+  summary('C Preprocessor Defines', run_command(cc, get_option('c_args') + simde_c_args + simde_c_defs + simde_native_c_flags + ['-dM', '-E', files('common/common.c')], capture: true, check: false).stdout())
+elif cc.get_id() == 'msvc'
+  summary('C Preprocessor Defines', run_command(cc, get_option('c_args') + simde_c_args + simde_c_defs + simde_native_c_flags + ['/PD', '/Zc:preprocessor', files('common/common.c')], capture: true, check: false).stdout())
+endif