Fix NMODL install paths

[JCGoran]

Fixes #3580.

Note that the various prefix forward slashes have been removed because if the preceding variable is empty, it will try to install it to the absolute path (for instance, /bin instead of bin, and the latter gets expanded to ${CMAKE_INSTALL_PREFIX}bin, where CMAKE_INSTALL_PREFIX already takes care of the paths).

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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[azure-pipelines]

✔️ 3d69b5e -> Azure artifacts URL

[github-actions]

✔️ 3d69b5e -> artifacts URL

[nrnhines]

Assuming a trailing '/' on path variables seems strange to me. Wouldn't an alternative be to check the variable and demand it be non empty?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.46%. Comparing base (0d99051) to head (3d69b5e).
⚠️ Report is 45 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3581   +/-   ##
=======================================
  Coverage   68.46%   68.46%           
=======================================
  Files         685      685           
  Lines      116720   116720           
=======================================
+ Hits        79914    79918    +4     
+ Misses      36806    36802    -4     

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[JCGoran]

Assuming a trailing '/' on path variables seems strange to me. Wouldn't an alternative be to check the variable and demand it be non empty?

Those are already taken care of, all custom paths that we set have a trailing slash (except when calling install(... DESTINATION ...) since there it doesn't matter):

nrn/CMakeLists.txt

Lines 43 to 58 in 3d69b5e

	if(SKBUILD)
	set(NRN_INSTALL_PYTHON_PREFIX "neuron/")
	set(NRN_INSTALL_DATA_PREFIX "neuron/.data/")
	# need to force fmtlib install prefixes
	set(FMT_LIB_DIR "${NRN_INSTALL_DATA_PREFIX}/lib")
	set(FMT_INC_DIR "${NRN_INSTALL_DATA_PREFIX}/include")
	set(FMT_PKGCONFIG_DIR "${NRN_INSTALL_DATA_PREFIX}/lib/pkgconfig")
	set(FMT_CMAKE_DIR "${NRN_INSTALL_DATA_PREFIX}/lib/cmake/fmt")
	# need to force IV install prefixes
	set(IV_LIB_INSTALL_DIR "${NRN_INSTALL_DATA_PREFIX}/lib")
	set(IV_HEADERS_INSTALL_DIR "${NRN_INSTALL_DATA_PREFIX}/include")
	set(IV_BIN_INSTALL_DIR "${NRN_INSTALL_DATA_PREFIX}/bin")
	else()
	set(NRN_INSTALL_PYTHON_PREFIX "lib/python/neuron/")
	set(NRN_INSTALL_DATA_PREFIX)
	endif()

The case of DESTINATION being empty in a given call to install is handled easily: cmake --install will fail, and we will get CI errors.