Strip GCC-only -fno-merge-constants from flags in conda build

[antonwolfy]

conda-forge's gcc/g++ compiler activation injects -fno-merge-constants into CFLAGS/CXXFLAGS when CONDA_BUILD==1.

The dpnp conda recipe builds with the Intel oneAPI compilers (icx/icpx), which do not implement this GCC flag and emit a warning on every compile unit:

icpx: command line warning #10430: Unsupported command line options encountered
These options as listed are not supported.
For more information, use '-qnextgen-diag'.
option list:
-fno-merge-constants

This PR removes -fno-merge-constants from CFLAGS/CXXFLAGS in conda-recipe/build.sh.

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/pull/2978/index.html

[coveralls]

coverage: 78.073%. remained the same — strip-gcc-no-merge-constants-flag into master

[github-actions]

Array API standard conformance tests for dpnp=0.21.0dev2=py313h509198e_5 ran successfully.
Passed: 1356
Failed: 4
Skipped: 16