and how to set your own picture

rather than dummy "main".
this commit is really UNTESTED on relevant platforms (w32)

rather than dummy "main"

rather than !HAVE_LIBGLEW.
this is necessary after updating the GEM_CHECK_LIB.

check: why is there no HAVE_GEM_LIB_GLEW or the like?

PKG_FOO is used in messages.
PKG_FOO_PKG_ERRORS is used to store per-pkg errors (?)

not needed any more (has it ever been needed?)

rather than on the library check

and use plural for HEADERS parm

(leave it blank for now)

and the same for -libs

LATER: should we append the additional search/include paths to the PKG_FOO_*
rather than overwrite it?

currently it's a no-op...

LATER use it, to check whether all required header-files are available,
in case pkg-config fails

if only the library is installed (but not the dev-headers),
GEM_CHECK_LIB will return SUCCESS (as it succeeds to link and does
not check for headers).

LATER: add an (optional) "headers" field to GEM_CHECK_LIB to centralize this

makeFont() must free all allocated ressources, so we don't need
to free m_font ourselves (which would result in a double free if
m_font is really only a proxy to the real font-ressource)

github Closes: umlaeute/Gem#16

and added dummy (t_symbol*) argument for setMess(), since CPPEXTERN_MSG()
requires it...