Make regex-pcre an optional dependency

This patch makes regex-pcre optional, allowing its disable via a
preprocessor define NO_REGEX_PCRE. This define will be added to
config.ac/Makefile.am in the next patch.

The patch also changes multi-line strings into string concatenation,
due to limitations with the CPP language extension.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>