Only build against prebuilt sysroot when using clang

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index 9c226a2fb71f02fa6c50fbaa8538be2def5213ee..8b59ba1e14157fa6b95511ec7a4f924e1a07763a 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -94,6 +94,12 @@
           # if packaging for a linux distro)
           'use_sysroot%': 1,
 
+          # Set this to true when building with Clang.
+          # See http://code.google.com/p/chromium/wiki/Clang for details.
+          # If this is set, clang is used as both host and target compiler in
+          # cross-compile builds.
+          'clang%': 0,
+
           # Override buildtype to select the desired build flavor.
           # Dev - everyday build for development/testing
           # Official - release build (generally implies additional processing)
@@ -153,6 +159,7 @@
         # Copy conditionally-set variables out one scope.
         'chromeos%': '<(chromeos)',
         'chromecast%': '<(chromecast)',
+        'clang%': '<(clang)',
         'desktop_linux%': '<(desktop_linux)',
         'use_aura%': '<(use_aura)',
         'use_ash%': '<(use_ash)',
@@ -299,7 +306,7 @@
           # Not used when chromecast=1 since ozone_platform_gbm doesn't
           # currently build against the linux sysroot
           # TODO(sbc): http://crbug.com/559708
-          ['OS=="linux" and chromeos==0 and chromecast==0 and use_sysroot==1', {
+          ['OS=="linux" and chromeos==0 and chromecast==0 and clang==1 and use_sysroot==1', {
             # sysroot needs to be an absolute path otherwise it generates
             # incorrect results when passed to pkg-config
             'conditions': [
@@ -325,6 +332,7 @@
       # Copy conditionally-set variables out one scope.
       'chromeos%': '<(chromeos)',
       'chromecast%': '<(chromecast)',
+      'clang%': '<(clang)',
       'host_arch%': '<(host_arch)',
       'target_arch%': '<(target_arch)',
       'target_subarch%': '<(target_subarch)',
@@ -625,11 +633,6 @@
 
       # Clang stuff.
       'make_clang_dir%': 'third_party/llvm-build/Release+Asserts',
-      # Set this to true when building with Clang.
-      # See http://code.google.com/p/chromium/wiki/Clang for details.
-      # If this is set, clang is used as both host and target compiler in
-      # cross-compile builds.
-      'clang%': 0,
 
       # Use experimental lld linker instead of the platform's default linker.
       'use_lld%': 0,