Fix path specification passed to AC_CHECK_PROG when checking for FileCheck

[andidr]

The set of locations in which AC_CHECK_PROG looks for a program is passed to the macro as a colon-separated path specification.

However, in the invocation of AC_CHECK_PROG for FileCheck, this specification is preceded by an extra path = , which causes the first location to be interpreted as path = $CLANG_PREFIX/bin rather than $CLANG_PREFIX/bin. This causes the check to fail, even if the file $CLANG_PREFIX/bin/FileCheck exists and is executable.

Removing the prefix makes the check work as expected.

[andidr]

Just saw that somebody mentioned the same in https://github.com/bondhugula/pluto/issues/96.