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).

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 :-)

This allows us to take advantage of newer features when they are
available. However, builds on Travis still use -std=c89 to ensure C89
compatibility. Locally this can be achieved with either

* cmake -DCMAKE_C_STANDARD=90 ..
* cmake -DCMAKE_C_FLAGS='-std=c89' ..
* C_FLAGS='-std=c89' cmake ..

We also reorder the warning flags: they are all part of the standard
build now, but -Werror is only applied on Travis.

Fixes #892.

The multistresstest code has been made C89 compliant in the process.

Setup 4 configs: Linux/GCC, Linux/CLang, OSX CLang and mingw32
OSX allowed to fail, since it fails in the proj -VC step.