PROJ/travis/linux_gcc7, branch 6.3.1

Upgrade testing from trusty to xenial, and also:

2019-09-25T08:50:07+00:00

* From openjdk-7 via apt to openjdk-11 via Travis CI xenial's $JAVA_HOME
* From mingw 4.8 -> 5.3, as supported by xenial
* clang+llvm for csa kept at version 6.0.0, but for ubuntu-16.04
* Furthermore xenial implicitly has many upgrades, specifically:
  - gcc 4.8.4 -> 5.4.0
  - clang 4.0.0 -> 5.0.0
  - cmake 3.9.2 -> 3.12.4
  - python 3.4.3 -> 3.5.2
* cppcheck version kept at 1.61 (for trusty) due to many
  false positives with version 1.72 (for xenial)
* Community code contribution documentation simplified to remove
  version of cppcheck, as this may outdated with updates to scripts

Rearrange Travis CI scripts to be more modular

2019-09-24T23:16:27+00:00

* Split global before_install for apt and pip
* Use -qq option for apt-get, which implies -y
* Use list of targets with apt-get rather than one-per-line
* Use local variables for mingw32/install.sh
* Show version info for cppcheck, pip and python
* Simplify clang+llvm-6 path

Require C99 compiler see #1621

2019-09-19T13:25:09+00:00

cmake and autoconf now stipulate C99
change c89 to c99 in travis jobs
remove HAVE_C99_MATH checks

(unrelated) relax Visual Studio compatibility check in
cmake/project-config-version.cmake.in (VS 2019 can use a VS 2015 library
but not vice versa).

Travis: add testing of JSON output against schema and upgrade everything to Python3

2019-07-07T22:05:01+00:00

autoconf build: add a PROJ_DB_CACHE_DIR trick to speed-up builds

2019-04-28T20:09:03+00:00

If the PROJ_DB_CACHE_DIR environment variable is defined, then a
$(PROJ_DB_CACHED_DIR)/proj.db.sql.md5 file is used to determine if
the set of .sql files has changed since the last time. If not then
$(PROJ_DB_CACHED_DIR)/proj.db is directly used.
This can saved a few seconds when doing rebuilds.
This is a poor man equivalent of ccache for generating the database :-)

.travis.yml: add build with g++-7 and -D_GLIBCXX_ASSERTIONS (refs #1290)

2019-02-24T10:55:26+00:00