set up a minimal build environment when using system compiler

[boegel]

It should be safe to define $CC & co to the standard GCC compiler commands when using the system toolchain.

Not only will this fix the problem discussed in #3398, it also ensures that values for $CC & co that were defined outside of EasyBuild don't mess things up...

(fixes #3398)

[lexming]

LGTM

[lexming]

Positive test with M4-1.4.18.eb in a sane environment: https://gist.github.com/lexming/9d1de285e054fc3d11605af6ef374422

== 2020-08-08 12:04:38,209 filetools.py:456 INFO Command gcc found at /usr/bin/gcc
== 2020-08-08 12:04:38,210 toolchain.py:257 INFO Found compiler command gcc at /usr/bin/gcc, so setting $CC in minimal build environment
== 2020-08-08 12:04:38,210 filetools.py:456 INFO Command g++ found at /usr/bin/g++
== 2020-08-08 12:04:38,210 toolchain.py:257 INFO Found compiler command g++ at /usr/bin/g++, so setting $CXX in minimal build environment
== 2020-08-08 12:04:38,211 environment.py:97 INFO Environment variable CC set to gcc (previously undefined)
== 2020-08-08 12:04:38,211 environment.py:97 INFO Environment variable CXX set to g++ (previously undefined)

[lexming]

Positive test with M4-1.4.18.eb in a environment without system compilers: https://gist.github.com/lexming/8202fedd8feab77ae50a7d7b18a0e9f8

== 2020-08-08 12:13:41,040 filetools.py:469 WARNING Could not find command 'gcc' (with permissions to read/execute it) in $PATH (['/user/brussel/101/vsc10122/src/easybuild-framework', '/user/brussel/101/vsc10122/bin', '/usr/sbin', '/usr/local/bin', '/usr/local/sbin', '/opt/ibutils/bin'])
== 2020-08-08 12:13:41,040 build_log.py:367 WARNING gcc command not found, not setting $CC in minimal build environment
== 2020-08-08 12:13:41,041 filetools.py:469 WARNING Could not find command 'g++' (with permissions to read/execute it) in $PATH (['/user/brussel/101/vsc10122/src/easybuild-framework', '/user/brussel/101/vsc10122/bin', '/usr/sbin', '/usr/local/bin', '/usr/local/sbin', '/opt/ibutils/bin'])
== 2020-08-08 12:13:41,041 build_log.py:367 WARNING g++ command not found, not setting $CXX in minimal build environment
[...]
configure: error: no acceptable C compiler found in $PATH
[...]

[lexming]

Positive test with foss-2019b.eb where it picks the compilers loaded by EB: https://gist.github.com/lexming/7273065257377601c25905b6bfd14b9e

== 2020-08-08 12:00:00,928 filetools.py:456 INFO Command gcc found at /apps/brussel/CO7/skylake/software/GCCcore/8.3.0/bin/ gcc
== 2020-08-08 12:00:00,928 toolchain.py:257 INFO Found compiler command gcc at /apps/brussel/CO7/skylake/software/GCCcore/8.3.0/bin/gcc, so setting $CC in minimal build environment
== 2020-08-08 12:00:00,930 filetools.py:456 INFO Command g++ found at /apps/brussel/CO7/skylake/software/GCCcore/8.3.0/bin/g++
== 2020-08-08 12:00:00,930 toolchain.py:257 INFO Found compiler command g++ at /apps/brussel/CO7/skylake/software/GCCcore/8.3.0/bin/g++, so setting $CXX in minimal build environment
== 2020-08-08 12:00:00,930 environment.py:97 INFO Environment variable CC set to gcc (previously undefined)
== 2020-08-08 12:00:00,931 environment.py:97 INFO Environment variable CXX set to g++ (previously undefined)

[boegel]

(just for the record: also tested with easybuilders/easybuild-easyconfigs#11122, which is a bit of a special case)

[casparvl]

I'm happy now :)