Stop referring to a removed make variable

Nothing uses the make variable GCCVERSION any longer, since we just
query the NDK for the default compiler.
improve perf
Add x86 32/64bit asm code for SumOfBlocks.
correct enc console help info
Check for $(CXX).exe as well when checking for the android toolchain

This fixes building for android on windows.

Building for android on windows still requires MSYS for running make,
but running the ndk-build of the sample projects from within MSYS
doesn't seem to work (the OS variable from the OpenH264 makefile
interferes with the host system detection in ndk-build), so one still
needs to build it in two steps, first make OS=android ... libopenh264.so,
then building the sample projects manually.
Merge pull request #1267 from zhilwang/sumofblocks_scc

Add x86 32/64bit asm code for SumOfBlocks.
Fix build error ARM
Merge pull request #1271 from dongzha/fixBuildError

Fix build error ARM
Add function define and init for SCC hash function
Merge branch 'master' of https://github.com/cisco/openh264 into x86_hash_scc
refine arm code for sum of frame
Merge pull request #1272 from zhilwang/x86_hash_scc

Add function define and init for SCC hash function
enable api test for decoder VclNal
Merge pull request #1274 from huili2/test_VclNal

enable api test for decoder VclNal
Merge pull request #1269 from huili2/enc_console_help

correct enc console help info
Add x86 32/64bit asm code for Scc_hash
Refine asm code format
Merge pull request #1273 from dongzha/RefineArmCodeForSumBlock

refine arm code for sum of frame
Merge pull request #1266 from mstorsjo/fix-readme

Stop referring to a removed make variable
Merge pull request #1270 from mstorsjo/android-tool-detect

Check for $(CXX).exe as well when checking for the android toolchain