PROJ/travis/linux_gcc7/install.sh, branch 7.0

Travis: update linux_gcc7 to linux_gcc8

2020-02-08T14:40:06+00:00

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).

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