add a macro that really identifies glibc

uClibc pretends to be glibc, so just checking for __GLIBC__ doesn't always
work.  Rather than check for defined(__GLIBC__) && !defined(__UCLIBC__) in
multiple places, do it once and define LIBC_GLIBC if we're certain that we're
really using glibc.

NOTRY=true

Review URL: https://chromiumcodereview.appspot.com/15405003

git-svn-id: http://src.chromium.org/svn/trunk/src/build@201761 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

build_config.h

@@ -34,6 +34,10 @@
 #if !defined(TOOLKIT_VIEWS) && defined(USE_X11)
 #define TOOLKIT_GTK
 #endif
+#if defined(__GLIBC__) && !defined(__UCLIBC__)
+// we really are using glibc, not uClibc pretending to be glibc
+#define LIBC_GLIBC
+#endif
 #elif defined(_WIN32)
 #define OS_WIN 1
 #define TOOLKIT_VIEWS 1