fixes #20

* PIE,RELRO+NOW and other security features enabled
* support for both static/dynamic builds with BUILDMODE
* dynamic library is ABI-versioned, starting at 1
* pkg-config file is installed

this allows to override any dstdir variable without
patching config.mk

amalgamated build concatenates all files into a single .c file to
allow compiler see all symbols and produce possibly smaller code.
for binary distributions this is what you want, as it's faster but
may consume more memory during compilation.
it however cannot do incremental builds.

the current Go doesn’t support c-shared on other architectures than amd64

same for LDFLAGS. this fixes common problem where CFLAGS after make
was evaluated as a make variable and replaced the default, while
before as env variable and was prepended to the default string.
now they both behave as an env. variable

this also fixes static linking missing a math library

This reverts commit 3f4d8cbe.

this saves space for both library and daemon

Headers were installed with executable flag set on Linux.

the build system now doesn't always relink targets