pkg-config file updates: 'Libs: -lm' + no '/signal' in 'Cflags: -I'

[fbrausse]

Hi,

when linking using the pkg-config file, -lm is missing. As far as I can see it is used only once:

libsignal-protocol-c/src/hkdf.c

Line 109 in 71954c5

int iterations = (int)ceil((double)output_len / (double)HASH_OUTPUT_SIZE);

This patch adds -lm to the pkg-config file.

It also removes the trailing path component /signal for the -I option in libsignal-protocol-c.pc.in, as this leads to "namespace pollution" in the sense that every pkg-config user of this library cannot use the file names of any header file libsignal-protocol-c provides (which are quite many). Leaving the path prefixed by signal/ on the client side is cleaner (and also what other libraries' pkg-config file exports, e.g., OpenSSL has its headers in include/openssl/ but does not include the suffix in its -I option).

Thanks for your consideration.