Fix some issues reported by lintian

Fix some issues reported by lintian

• Trim trailing whitespace. (trailing-whitespace)
• Use secure URI in Homepage field. (homepage-field-uses-insecure-uri)
• Bump debhelper from deprecated 7 to 13. (package-uses-deprecated-debhelper-compat-version)
• Set debhelper-compat version in Build-Depends. (uses-debhelper-compat-file)
• Set upstream metadata fields: Bug-Database, Bug-Submit (from ./configure), Name (from ./configure), Repository-Browse. (upstream-metadata-file-is-missing, upstream-metadata-missing-bug-tracking)
• Drop unnecessary dependency on dh-autoreconf. (useless-autoreconf-build-depends)
• Add patch ac-path-pkgconfig.patch: Use cross-build compatible macro for finding pkg-config. (autotools-pkg-config-macro-not-cross-compilation-safe)

Debdiff

These changes affect the binary packages:

[The following lists of changes regard files as different if they have different names, permissions or owners.]

Files in second set of .debs but not in first

-rw-r--r--  root/root   /usr/lib/debug/.build-id/c5/ac74533d65c77e74bcc3f20f257a70d2f5b801.debug

Files in first set of .debs but not in second

-rw-r--r--  root/root   /usr/lib/debug/.build-id/c6/5910821c82ec3ff7b94bcccb7fde2f98dda870.debug

Control files of package libgdiplus: lines which differ (wdiff format)

• Depends: libc6 (>= 2.29), libcairo2 (>= 1.10.0), libexif12 (>= 0.6.21-1~), libfontconfig1 (>= 2.12.6), libgif7 (>= 5.1), libglib2.0-0 (>= 2.31.8), libharfbuzz0b (>= 2.6.0), libjpeg62-turbo (>= 1.3.1), libpango-1.0-0 (>= 1.44.3), libpangocairo-1.0-0 (>= 1.18.0), libpng16-16 (>= 1.6.2-1), libtiff5 libtiff6 (>= 4.0.3), libx11-6
• Homepage: http​://www.mono-project.com/Libgdiplus https​://www.mono-project.com/Libgdiplus

Control files of package libgdiplus-dbgsym: lines which differ (wdiff format)

• Build-Ids: c65910821c82ec3ff7b94bcccb7fde2f98dda870 c5ac74533d65c77e74bcc3f20f257a70d2f5b801

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Build and test logs for this branch can be found at https://janitor.debian.net/run/cd7157c8-2a9e-4b39-af2d-cd61bbebafa7.