Alter the path to version.h.

[whitwham]

This prevents the compiler from picking up htslib's version.h when C_INCLUDE_PATH=: (or similar).

From #817:
Simplest way to reproduce the error:

tar xvfj samtools-1.8.tar.bz2
cd samtools-1.8
./configure
C_INCLUDE_PATH=: make

The : in the C_INCLUDE_PATH is in effect adding -I. to the end of the gcc list of include paths while removing the original -I. from the start.
So
gcc -g -O2 -DMAQ_LONGREADS -I. -Ihtslib-1.8 -I./lz4 -c -o misc/maq2sam-long.o misc/maq2sam.c
becomes
gcc -g -O2 -DMAQ_LONGREADS -Ihtslib-1.8 -I./lz4 -I. -c -o misc/maq2sam-long.o misc/maq2sam.c

Though there is no apparent change in the command line, internally gcc removes any duplicate paths that are also in C_INCLUDE_PATH. This makes gcc find the htslib version.h before the samtools version.h so the build fails with a missing SAMTOOLS_VERSION.

The gcc and clang compilers both exhibit this behaviour.

Fixes #817.