symbol-scan.pl: do not require autotools

Makes test1119 pass when building with cmake.

configurehelp.pm is generated by configure (autotools). As cmake does
not provide a separate variable for the C preprocessor, default to cpp.
Before commit ef24ecde

 ("symbol-scan:
use configure script knowledge about how to run the C preprocessor"),
this tool would also use 'cpp'.

Signed-off-by: Peter Wu <peter@lekensteyn.nl>