configure: don't pass compile flags through WRAPPER

[hzhou]

Pull Request Description

PAC_PREFIX_FLAG previously copied user's flags (e.g. CFLAGS) into WRAPPER_CFLAGS, causing mpicc to pass them to user applications. WRAPPER compile flags should only contain flags explicitly needed by applications, not MPICH's own build flags.

• Change PAC_PREFIX_FLAG to initialize prefixed variables to empty
• Remove -g, sanitizer, and coverage flag appends to WRAPPER flags
• WRAPPER_LIBS is unaffected and continues to pass needed libraries

tag @Growl1234

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[hzhou]

test:mpich/ch3/most
test:mpich/ch4/most

[hzhou]

test:mpich/ch3/most
test:mpich/ch4/most

[Growl1234]

Correct me if I'm wrong (this comment is generated with the help of AI):

This fixes the immediate leakage of CFLAGS, CXXFLAGS, FCFLAGS, etc. into the compiler wrappers. However, PAC_PREFIX_FLAG is also used by PAC_PREFIX_ALL_FLAGS(USER). With the new semantics, PAC_PREFIX_ALL_FLAGS(USER) initializes all USER_* variables to empty instead of saving the user-provided flags. These variables are subsequently used by PAC_RESET_ALL_FLAGS and PAC_RESET_LINK_FLAGS, so this may cause subconfigures to lose the original build flags.

[hzhou]

@Growl1234 Thanks for the good catch! You are right.