Makefile: introduce debugsymbols=on flag

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/1085283002

Cr-Commit-Position: refs/heads/master@{#27852}

Makefile

@@ -97,6 +97,10 @@ endif
 ifeq ($(slowdchecks), off)
   GYPFLAGS += -Dv8_enable_slow_dchecks=0
 endif
+# debugsymbols=on
+ifeq ($(debugsymbols), on)
+  GYPFLAGS += -Drelease_extra_cflags=-ggdb3
+endif
 # gdbjit=on/off
 ifeq ($(gdbjit), on)
   GYPFLAGS += -Dv8_enable_gdbjit=1

build/standalone.gypi

@@ -41,6 +41,7 @@
     'v8_deprecation_warnings': 1,
     'msvs_multi_core_compile%': '1',
     'mac_deployment_target%': '10.5',
+    'release_extra_cflags%': '',
     'variables': {
       'variables': {
         'variables': {
@@ -196,7 +197,7 @@
         # Xcode insists on this empty entry.
       },
       'Release': {
-        # Xcode insists on this empty entry.
+        'cflags+': ['<@(release_extra_cflags)'],
       },
     },
     'conditions':[