Fix build for UCLIBC platforms

malloc_usable_size does not exist in UCLIBC, so fall back to just
returning 0 for SkVarAlloc::heap_size().

BUG=skia:

Review URL: https://codereview.chromium.org/1006073003

src/core/SkVarAlloc.cpp

@@ -47,7 +47,7 @@ void SkVarAlloc::makeSpace(size_t bytes, unsigned flags) {
 
 #if defined(SK_BUILD_FOR_MAC)
     SkASSERT(alloc == malloc_good_size(alloc));
-#elif defined(SK_BUILD_FOR_UNIX)
+#elif defined(SK_BUILD_FOR_UNIX) && !defined(__UCLIBC__)
     // TODO(mtklein): tune so we can assert something like this
     //SkASSERT(alloc == malloc_usable_size(fBlock));
 #endif
@@ -56,7 +56,7 @@ void SkVarAlloc::makeSpace(size_t bytes, unsigned flags) {
 static size_t heap_size(void* p) {
 #if defined(SK_BUILD_FOR_MAC)
     return malloc_size(p);
-#elif defined(SK_BUILD_FOR_UNIX)
+#elif defined(SK_BUILD_FOR_UNIX) && !defined(__UCLIBC__)
     return malloc_usable_size(p);
 #elif defined(SK_BUILD_FOR_WIN32)
     return _msize(p);

tests/VarAllocTest.cpp

@@ -6,8 +6,8 @@ DEF_TEST(VarAlloc, r) {
     char* p = va.alloc(128, SK_MALLOC_THROW);
     sk_bzero(p, 128);  // Just checking this is safe.
 
-#ifndef SK_BUILD_FOR_ANDROID
+#if !defined(SK_BUILD_FOR_ANDROID) && !defined(__UCLIBC__)
-    // This method will always return 0 on Android.
+    // This method will always return 0 on Android and UCLIBC platforms.
     REPORTER_ASSERT(r, va.approxBytesAllocated() >= 128);
 #endif
 }