Support using ccache when building gengetopt and ragel

[gavv]

User can enable ccache using --compiler-launcher=ccache scons option.

When user specifies --build-3rdparty=<dependencies>, and ccache is enabled, we automatically use ccache for almost all dependencies. However, we don't enable ccache for gengetopt and ragel. See build-3rdparty.py script.

The reason is that gengetopt and ragel are tools needed during build. Unlike other dependencies, they're not cross-compiled. Instead, they are built natively and then scons is using them. Because of that, build-3rdparty.py doesn't provide CC and CXX vars when it builds gengetopt and ragel, and so there is no place where it could inject ccache.

So the fix is straightforward:

• teach build-3rdparty.py to detect CC and CXX for gengetopt and ragel (it should check if gcc/clang/cc is available in PATH and use it)
• if ccache is enabled (i.e. COMPILER_LAUNCHER is set), add it to CC and CXX (e.g. CC=gcc becomes CC="ccache gcc")

See also #638.

[jeshwanthreddy13]

Hi @gavv , I'm interested in this project and would like to get involved. Could you assign this issue to me so I can start working on it?

[gavv]

Welcome, thanks!

[gavv]

Landed