Fix compiler flags issue

[JcZou]

CCFLAGS is used by gcc and g++ compiler. So CFLAGS should be used for gcc to avoid passing gcc flags to g++.

拉取/合并请求描述：(PR description)

[
Fix the wrong compiler flags for gcc.

CCFLAGS is general options that are passed to the C and C++ compilers. CFLAGS should should be used for gcc as it's the options that are passed to the C compiler only.

Has been tested on stm32 bsp.
]

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[mysterywolf]

#2855

[JcZou]

CCFLAGS += ' -std=c99'
CXXFLAGS += ' -std=c++14'
最终的g++的flag将是两者相加, g++将报无法识别'-std=c99的警告'

[Guozhanxin]

这么改的话，开启c++是不是会有问题啊，感觉后面ci又要加上c++的测试了😂

[JcZou]

这么改的话，开启c++是不是会有问题啊，感觉后面ci又要加上c++的测试了😂

如果rtconfig.py的gcc编译是这样写的就没问题（c++的std选项可以没有）
CXXFLAGS = CFLAGS
CFLAGS += ' -std=c99'
CXXFLAGS += ' -std=c++14'

[Guozhanxin]

您好，这个pr有冲突了，您有时间的话就解决一下，解决之后就可以合并了。

[liukangcc]

由于作者长期未回复，该 PR 已重新提交：#5171

[mysterywolf]

作者还在睡觉😂 现在是欧洲凌晨