configure.ac: use MbedTLS pkg-config if available

[pulsastrix]

Newer MbedTLS versions provide pkg-config files for easier dependency management/linking, see the release notes of versions 3.6.0 and 2.28.8.

This PR allows the autoconf build script to utilize the mbedtls pkg-config file if it exists, falling back to the previous method of finding MbedTLS if no pkg-config file was found.

Additionally, this change allows overriding MbedTLS_CFLAGS and MbedTLS_LIBS, which was previously not possible (unlike all the other DTLS libraries).

[mrdeep1]

Thanks for doing this.

Great that MBed TLS have eventually added .pc files. Shame that they do not have it for their make system, only for cmake.

However, for libcoap

./configure --with-mbedtls --disable-doxygen --disable-manpages --enable-tests --disable-shared
make

fails with

make[2]: Entering directory '/home/jon/libcoap/tests'
  CCLD     testdriver
/usr/bin/ld: ../.libs/libcoap-3-mbedtls.a(libcoap_3-mbedtls_la-coap_mbedtls.o): undefined reference to symbol 'mbedtls_x509_crt_parse_file'
/usr/bin/ld: /usr/local/lib/libmbedx509.so.7: error adding symbols: DSO missing from command line

or

./configure --with-mbedtls --disable-doxygen --disable-manpages --disable-tests --disable-shared
make

gives

  CCLD     coap-client-mbedtls
/usr/bin/ld: ../.libs/libcoap-3-mbedtls.a(libcoap_3-mbedtls_la-coap_mbedtls.o): undefined reference to symbol 'mbedtls_x509_crt_parse_file'
/usr/bin/ld: /usr/local/lib/libmbedx509.so.7: error adding symbols: DSO missing from command line

The issue here is the non-shared file ../.libs/libcoap-3-mbedtls.a does not indicate that the libraries mbedcrypto mbedx509 are not indicated as being needed.

./configure --with-mbedtls --disable-doxygen --disable-manpages --disable-tests --enabled-shared
make

does work as expected (note: tests/testdriver has to use the static file, hence why that is always failing).

I suspect that you may need to use PKG_CHECK_MODULES_STATIC if --disable-shared is set.

[pulsastrix]

Hi, thanks for the review.

I have now replaced PKG_CHECK_MODULES with PKG_CHECK_MODULES_STATIC if --enable-static is set, which should fix the issue you mentioned (at least it did on my system).

[mrdeep1]

@pulsastrix It is looking good. Just a couple of things.