'syntax-check' minor portability issues

[Assaf Gordon]

Hello,

two rules in 'syntax-check' fail with non-gnu tools:

sc_long_lines (cfg.mk): uses 'wc -L' and 'sed -r' - not supported on BSD's 
wc/sed.
perhaps it should even use '$(SED)' instead of 'sed'?

_gl_tight_scope (gnulib/top/maint.mk): uses 'nm -e', '-e' not supported with 
Apple Xcode's nm, and is silently ignored in GNU nm.
perhaps the '-e' can simply be removed?

The attached patch skips sc-long-lines if needed (note: minimally tested on Mac 
OS X 10.9.5 with Xcode 6.1, and on Debian 7 64bit).

- Assaf

sc_long_lines.nongnu.patch
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