Scons fails to detect libpng library on CentOS 6

[tst-ccamp]

This issue comes up a lot because a default g++ compiler is not installed when following the build setup instructions for CentOS 6. I don't believe this issue comes up in RHEL 6 because it's included with a documented package. Furthermore, even when fixing the issue it still will need to rescan for the configs (--config=force on the scons command line). This should happen by default since the time to do the scan is negligible.

Reproduction steps

1. Create CentOS 6 minimal VM
2. Follow manual build setup instructions
   1.Build using scons

Expected: build completes without error

Actual: Error message saying that it can't find libpng12

AC

1. Update build instructions to include default g++ for CentOS 6 (gcc-g++ package) or update scons to use devtools g++ install
2. Add a check to scons for a g++ compiler and output an appropriate error if it can't find one
3. Update scons to always scan for libpng (it may be possible to disable caching) when running the third_party tools build

[tst-ppenev]

Have you timed how long it takes to rescan for all the libraries used in a build?

I think the proper fix is to update SCons scripts to report a proper error when they can't find a compiler, or a library (e.g., 'Install g++.', 'Install libpng headers, and rebuild with scons --config=force.'), instead of making strange assumptions, continuing, and outputting hard to decipher messages.

[tst-ccamp]

I didn't bother timing because it's probably less time than it takes to print the message to the console. But, I add an AC to do a proper check for the g++ compiler.

[tst-ppenev]

Scanning for libraries involves running GCC for every header, and dynamic library you're testing for. That usually takes, at least, a few minutes for all the libraries used by GEE.

[tst-ccamp]

I ran some tests. It looks like the --config=force flag works well for the third_party target, but I got an error for the default build target. Here are the results on the third_party target. If we just stick to that, I don't think we'll add to the total build time. And the build will stop if it can't find the library on the third_party target.

ccamp@Mainframe:/opt/dev/tst/earthenterprise/earth_enterprise/src$ time scons -j12 release=1 --config=force third_party
scons: Reading SConscript files ...

scons: warning: The build_dir keyword has been deprecated; use the variant_dir keyword instead.
File "/opt/dev/tst/earthenterprise/earth_enterprise/src/SConstruct", line 507, in <module>
Checking for C library gtest... yes
...
scons: done building targets.

real	0m4.143s
user	0m4.104s
sys	0m0.168s

ccamp@Mainframe:/opt/dev/tst/earthenterprise/earth_enterprise/src$ time scons -j12 release=1  third_party
scons: Reading SConscript files ...

scons: warning: The build_dir keyword has been deprecated; use the variant_dir keyword instead.
File "/opt/dev/tst/earthenterprise/earth_enterprise/src/SConstruct", line 507, in <module>
Checking for C library gtest... (cached) yes
...
scons: done building targets.

real	0m4.122s
user	0m4.068s
sys	0m0.192s

[tst-ppenev]

👍 Looks good to me.