pass down compilation flags from build environment for ESMF

[lexming]

The build of ESMF uses its own set of flags and mostly ignores the standard flags in the environment. This PR passes the flags from EB to the build of ESMF if optarch is set in toolchainopts.

Building ESMF with EB flags vastly improves the number of tests that pass (make unit_tests):

• current develop: Found 3478 non-exhaustive multi-processor unit tests, 285 passed and 3193 failed
• this PR: Found 3478 non-exhaustive multi-processor unit tests, 3209 passed and 269 failed.

Update: after further tests, the improvements seem to boil down to having a proper optarch

• setting ESMF_F90COMPILEOPTS=-O2 -ftree-vectorize -fno-math-errno -fPIC results in 229 passed and 3249 failed unit tests
• setting ESMF_F90COMPILEOPTS=-O2 -ftree-vectorize -march=native -fno-math-errno -fPIC results in 3209 passed and 269 failed unit tests

[lexming]

Successful tests with this PR:

• ESMF v7.1.0r in foss/2019a: {geo}[foss/2019a] ESMF v7.1.0r easybuild-easyconfigs#12060 (comment)
• ESMF v8.0.0 in foss/2019b and intel/2019b: {geo}[foss/2019b] ESMF v8.0.0 easybuild-easyconfigs#12069 (comment)
• ESMF v8.0.1 in foss/2020a and foss/2020b: {geo}[foss/2020b] ESMF v8.0.1 easybuild-easyconfigs#12070 (comment)

[smoors]

lgtm

[smoors]

Going in, thanks @lexming!

[lexming]

Test report by @lexming

Overview of tested easyconfigs (in order)

• SUCCESS ESMF-7.1.0r-iomkl-2018b.eb
• SUCCESS ESMF-7.1.0r-intel-2018b.eb
• SUCCESS ESMF-7.1.0r-intel-2018a.eb
• SUCCESS ESMF-7.1.0r-foss-2018b.eb
• SUCCESS ESMF-7.0.2-intel-2017b.eb
• SUCCESS ESMF-6.3.0rp1-intel-2017a-HDF5-1.8.18.eb
• SUCCESS ESMF-6.3.0rp1-intel-2017a.eb

Build succeeded for 7 out of 7 (7 easyconfigs in total)
node379.hydra.os - Linux centos linux 7.9.2009, x86_64, Intel(R) Xeon(R) Gold 6148 CPU @ 2.40GHz, Python 2.7.5
See https://gist.github.com/8c01f0c045e7f34065d4915d3267b20a for a full test report.