Verify OpenSSL even if custom location is requested

[nnposter]

The patch is intended to fix #2420. Specifically, it implements these changes:

• Moves existing AC_SEARCH_LIBS(dlopen, dl) forward, so that libdl is available for linking when OpenSSL tests are carried out.
• Performs existing OpenSSL header and library tests even if custom library location is specified via --with-openssl=somedir.
• Prints appropriate error messages if these tests fail, differentiating between --with-openssl=somedir and --with-openssl=yes

As a result:

• OpenSSL issues with custom library locations are detected earlier, at configure time, instead of during make.
• Presence or absence of ECC support is correctly configured (HAVE_OPENSSL_EC not set when building with local OpenSSL #2420)

Please review and comment. Note that the logical patch is rather tiny. Most of the delta is caused by different indentation of existing code.

If no issues are raised, this PR will be committed on or after March 1st.

[nnposter]

The fix has been committed as r38364.