Makefile: allow supplying custom flags via config.mak

Allow users to override the default flags by providing an untracked
"config.mak" file in the src/ directory.  This allows someone to have,
for example,

	CXXFLAGS += -Wsign-compare -Wconversion -Werror

in their config.mak and Ptex will build using those flags.

Signed-off-by: David Aguilar <davvid@gmail.com>
warnings: eliminate conversion warnings

This allows Ptex to be built using -Wconversion and -Wsign-compare.

Signed-off-by: David Aguilar <davvid@gmail.com>
Merge pull request #7 from davvid/warnings

Eliminate warnings when building with -Wsign-compare -Wconversion