libgd: avoid using host headers

[ryzhovau]

Maintainer: @jow-, @neheb
Compile tested: Clean Debian 10 stable host, Entware repo, mipsel feed (commit).
Run tested: Entware repo

Description: This PR removes host paths from CFLAGS, regression introduced after switch to CMake. Otherwise, libgd fails diring CMake tests:

make[5]: Entering directory '/home/Entware/build_dir/target-mipsel_mips 32r2_glibc-2.27/libgd-2.2.5/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_0b835.dir/HAVE_STDINT_H.c.o
/home/Entware/staging_dir/toolchain-mipsel_mips32r2_gcc-7.4.0_glibc-2.2 7/bin/mipsel-openwrt-linux-gnu-gcc  -I/usr/include -I/usr/local/include  -O2 -pi pe -mno-branch-likely -mips32r2 -mtune=mips32r2 -fno-caller-saves -fhonour-copts  -Wno-error=unused-but-set-variable -Wno-error=unused-result -msoft-float  -fpic  -ffunction-sections -fdata-sections -flto    -o CMakeFiles/cmTC_0b835.dir/HAVE_ STDINT_H.c.o   -c /home/Entware/build_dir/target-mipsel_mips32r2_glibc- 2.27/libgd-2.2.5/CMakeFiles/CheckIncludeFiles/HAVE_STDINT_H.c
In file included from /home/Entware/build_dir/target-mipsel_mips32r2_gl ibc-2.27/libgd-2.2.5/CMakeFiles/CheckIncludeFiles/HAVE_STDINT_H.c:2:0:
/usr/include/stdint.h:43:9: error: unknown type name '__int_least8_t'
 typedef __int_least8_t int_least8_t;
         ^~~~~~~~~~~~~~
/usr/include/stdint.h:44:9: error: unknown type name '__int_least16_t'
 typedef __int_least16_t int_least16_t;
         ^~~~~~~~~~~~~~~
/usr/include/stdint.h:45:9: error: unknown type name '__int_least32_t'
 typedef __int_least32_t int_least32_t;
         ^~~~~~~~~~~~~~~
/usr/include/stdint.h:46:9: error: unknown type name '__int_least64_t'
 typedef __int_least64_t int_least64_t;

[neheb]

Signed-off-by is missing or doesn't match author (should be 'Signed-off-by: Alexander Ryzhov github@ryzhov-al.ru')

[karlp]

yet MORE regressions from gratuitously changing build systems!